The labyrinth of gullies, the tunnels and mud-walled compounds; the thump of rockets and rattle of gunfire, the shouts and confusion ... in the words of the wounded soldiers it seemed like another small version of Hell in an Afghan firefight. The battalion's advance units were pinned down, taking heavy fire from unseen insurgents in multiple positions. Jets flew overhead, artillery was on standby, but the combat was too close-quarter for either to be used. A ten-man assault group led by a captain was caught in the open, lying in a wheatfield. Bullets scythed the crop around them.

An exploding rocket ripped open the captain's right side. His radio operator dropped wounded beside him. Dragged to an ambulance, they found their colonel already on a stretcher. One of his legs had been all but severed. He had been felled by a mortar round and shot twice. Four of his headquarters staff were wounded in the blast. Outside the ambulance, more explosions and more shooting followed. Now six of the ten-man assault group lay dead in the sheaves of wheat.

The battalion had begun its attack at 6am. The objective looked small, just a few mud-walled compounds in the village of Khazana. Little more than three hours later, seven soldiers were dead and 27 wounded.

But this was not Afghanistan; these were not British troops. Instead, just across the border, it was Pakistani soldiers giving their lives in a fight against the Taleban. And that day was far from the worst in the continuing Operation Sherdil: for the past three months, fighting along an eight-mile (13km) stretch of road in the tribal agency of Bajaur has claimed the lives of 83 troops and wounded 300. A total of 20 other soldiers are missing in action, presumed dead.

'After the suicide bomb our escort vehicle was hit'

Seen from the front line, it is difficult to question Pakistan's commitment to the bloody war against the Taleban Watch our video

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The card displayed beside the hospital bed of the officer who lost his leg that September day, Lieutenant Colonel Zahid Mehmood, was made by his two young sons. “Our Hero” it reads on the front, in English.

The room could be that of a British officer in Selly Oak, Birmingham,rather than the Combined Military Hospital in Peshawar, where Colonel Mehmood and Captain Inam-ur-Rahman spoke of the action; just two men among the many Pakistani soldiers wounded in the operation to date.

Standing at his bedside, Zahid's wife, Uzma, 33, cut a figure of stoicism. “We are not afraid of these sacrifices,” she said. “I believe he has done a noble job for his country and religion and I am proud of him.”

Uzma's words may give pause for thought among the chorus line of regional players - including Nato, the US and Britain - who have long questioned Pakistan's commitment in dealing with the militants in its seven Federally Administered Tribal Areas (Fata) bordering Afghanistan. Numerous previous operations conducted by Pakistan's military over the past five years in those areas have ended with flawed ceasefires, inconclusive negotiations and a resurgent Taleban. Many have accused Pakistan's intelligence service, the ISI, of conniving with the insurgents. “This could be the moment that ends the Pakistani military's denial of the problem they are facing in the Fata,” one Western diplomat remarked. “A watershed in which they realise just how serious the insurgency has become.”

Any doubts as to the gravity of the situation in Bajaur were quickly dispelled as The Times drove there. As we skirted the immediate aftermath of a suicide attack that killed nine people in the town of Mardan, our escort vehicle was hit by a roadside bomb. The device had been poorly positioned; as the seven Frontier Corps militiamen sprinted past us out of the smoke, it appeared that a hapless motorcyclist, carried bleeding from the scene, was the only casualty. It was still an hour's drive from the Fata borders and in a supposedly calm area.

In Bajaur itself, the hotly contested stretch of road, flanked by devastatedvillages, crackled with sporadic sniper and machinegun fire. A gunship strafed a distant slope; nearer, barely 100 yards from a Pakistani position on the road, mortar fire thumped into a compound. “We've just seen movement there,” a captain explained from behind a sandbagged wall as his radio operator adjusted the fire. “There aren't any civilians here and we are shot at from the house every night.”

Local media have called the operation the defining “do or die” moment for Pakistani security forces in the struggle against the militants. The US and Nato are watching just as closely. Given the task of securing the unconditional surrender of the Taleban in Bajaur, which borders Afghanistan, Major-General Tariq Khan, the Frontier Corps commander at the forefront of Operation Sherdil, is unequivocal. “We're not even doing this to lead to negotiation,” he told The Times at his headquarters in Peshawar.

“We're very clear. We've told them [the Taleban] it's just two or three options they have. They can surrender to the local commandant with their weapons. They will go to a proper court which we'll establish and they'll get their sentences. Or they can prepare to die.” Yet Pakistan's Government seems more ambivalent. Last week Parliament passed a 14-point resolution in which it recommended that the fighting should be ended through negotiation. The country remains deeply divided between those supporting military operations and those who believe that the violence has been visited upon them at America's behest.

The alleged hiding place of Ayman al-Zawahiri, alQaeda's second-in-command, Bajaur is a key transit route for Taleban passing in and out from other tribal areas as well as Afghanistan. The Army has only just cleared the insurgents from positions along the road linking the town of Loesam with Bajaur's capital, Khar; now it has bottled the Taleban into two valleys and is waiting for Nato to deploy forces to secure the Afghan border before further advances.

The operation is expected to last for up to two more months and the fiercest fighting could lie ahead. If pushed to its natural conclusion, Operation Sherdil might mark the belated recognition that Nato and the Pakistani army share a common enemy. But any wavering in resolve could undermine what has been achieved at such cost, and mark another low point in Pakistan's efforts to subdue the militants.

Fighting began there in August and caught the military by surprise. A Frontier Corps unit of 150 men was sent to establish a post at Loesam, where a crossroads links routes in four directions. Within two hours they found themselves under attack and were soon surrounded by a Taleban force estimated to be more than 1,000 strong.

Two attempts by troops based at Khar to relieve the encircled force failed after suffering heavy casualties.

With no water or rations left and their ammunition all but expended, on the fourth, rainy night the beleaguered force managed to break out. The last man to leave was their commander, Major Ijaz Hussain. Separated from his men, he spent two days hidden in a field of maize with one other soldier - the Taleban all around them. He was wounded by fire from a Pakistani gunship before making a successful escape. “I had two rounds left in my gun,” he said. “I was saving them for myself rather than be captured.”

Having taken Loesam, and scenting victory, the Taleban moved on quickly to encircle Khar. The Frontier Corps there were briefly surrounded in their crenellated fortress as insurgents from neighbouring tribal areas and Afghanistan poured in for the attack. Officers estimate that more than 4,000 insurgents were in Bajaur, of whom at least 1,000 had arrived from Afghanistan. They claim to have killed more than 1,500 in the past few weeks but admit - similar to Nato's claims in Afghanistan - to having recovered only “tens” of militant bodies.

Shocked by the strength of the Taleban in Bajaur and aware of a nationwide sense that the militants were in the ascendancy, thanks to a series of suicide-bomb attacks and assassinations, the Pakistani Army committed a brigade of reinforcements to Khar on September 9. Initially given the job of pushing up from Khar and recapturing Loesam, they immediately found themselves in pitched combat against a well-armed enemy equipped with mortars, rockets, heavy machineguns and satellite communications systems. Extensive tunnel complexes, piled with rations and munitions, linked a series of defensive positions.

For many of the regular troops, trained for conventional war with India, the new style of combat came as a brutal surprise. “It's a guerrilla war in a built-up area and forest, against a strongly held defence line held by people who are invisible,” said Lieutenant-Colonel Javed Baluch, in Tang Khata, a village captured by his men in September. “Many of the engagements are at 25 yards,” he added, visibly surprised by the nature of the action. “By the time you realise they are there, you are hit. It's a bloody organised group. Give them credit, they fight in an organised way.”

The Army's push was slow and bloody as the insurgents kept melting away and reappearing to ambush and harry the advance. Tang Khata, little more than a mile from Khar, took five days to subdue. It was ten days later that Colonel Mehmood lost his leg and seven men. Another battalion lost 12 dead and 42 wounded retaking the next village, Nisarabad.

As the Army advanced laboriously toward Loesam, which it finally recaptured on October 23, a series of lashkars - units of local tribesmen - were co-opted to join the Government's fight. Their support on the battlefield has been advertised as proof that the local populace has decisively rejected the presence of the Taleban, but it comes with many caveats. In one valley the lashkars have pushed out the Taleban, but in another, Charmung, they have been decisively beaten. In a third, Mamund, they have yet to organise.

More than 100,000 civilians remain displaced by the fighting. Most heeded the Government's call to leave Bajaur at the start of the operation. Officials state that 95 civilians have been killed in the fighting but the figures are impossible to verify. The Times spoke to numerous families who said that relatives had been killed by artillery or airstrikes even as they fled.

Jehnazeb, 27, from Loisam, left his home on foot along with 25 family members, including his six-month-old daughter, in September. Soon after they had begun the long journey to a refugee camp they were repeatedly bombed by a jet. “There was one huge blast and smoke everywhere,” he said. “We scattered and fled for our lives into a field of corn. For 25 minutes we were too frightened to go back to the track. When we did I found that my sister-in-law was blown to bits. My brother and two children were wounded. My mother had been cut in half. She was 67. We buried them then and there at the roadside.

“After a couple of hours we continued our journey to the camp. The wounded walked.”

Those who do return to Loesam one day will find that their homes have been reduced to a wasteland. In the centre of the town barely one brick sits upon another.

“The fight ahead will be different,” said Major Anwar Saeed, as he stepped over the rubble and looked toward the towering peaks beyond. “It'll be tougher. Mountain warfare. We'll change our tactics and the Taleban will change theirs.”

He paused for a second as a nearby tank engaged targets on the slopes. “In October 2001 we thought this would last just a few months. Now it's a few years. Militarise an area and it's easy to make a beast out of man - and so difficult to make a man out of a beast.”

Land of extremes

— The Federally Administered Tribal Areas (Fata) are semi-autonomous regions in which most laws passed by Pakistan's National Assembly do not apply. Instead, they are governed by tribal leaders and follow a mixture of tribal and Islamic laws and codes

— Bajaur covers an area of about 500 sq miles (1,290 sq km). Khar, its administrative centre, lies about 85 miles from the Pakistani city of Peshawar

— The terrain is ill-suited to military manoeuvres, with peaks ranging from 2,500m (8,200ft) in the south to 3,000m in the north. It is criss-crossed by rivers, forming deep valleys

— The climate is extreme, with temperatures from freezing in the winter to 36C in summer

— The two main tribes in the area are the Tarkanai and Uthman khel

Sources: Fata Secretariat; Times archive

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