Gazing across from a clifftop watchtower at a swath of territory controlled by the Taleban, the Prime Minister saw for himself at the weekend the rocky landscape around Musa Qala that is proving an increasing challenge for Britain's embattled contingent in Afghanistan. On Friday four British Marines were killed by a 13-year-old suicide bomber about 30 miles from the site of Gordon Brown's first frontline visit. Today, he will tell Parliament that a further 300 troops will be sent to Afghanistan. Next year two SAS squadrons are to be deployed there from Iraq to mount covert operations in Helmand province.

The pace of Nato operations is quickening. The Americans are preparing to send 20,000 more troops, most of them by the summer, to add to the 67,000-strong foreign forces in Afghanistan. But this increased backing for the International Security Assistance Force (Isaf) is barely enough to hold the Taleban at bay, let alone drive them away from Kabul and back towards Pakistan as the Nato strategy proposes. A spate of attacks on supply convoys to Nato and US forces has caused border closures and huge backlogs, with around 1,000 lorries stalled on the Afghan border. Pakistani militants set fire to at least 260 vehicles last week and attacked two cargo terminals in Peshawar. In attempting to strangle the foreign forces deployed in the capital, the Taleban are following the tactics used so effectively against the British in the 19th century and more recently against the Soviet Union.

Nato needs desperately to rethink its strategy and its logistics. It has already begun talks with Moscow and governments in Central Asia on opening new supply routes through Russia or Turkmenistan. These will be costlier, take longer and depend on Nato's attempts to repair its frayed relations with Russia. Without them, however, any attempt to scale up the fighting against the Taleban will be impossible. Already the Afghan Army carries out half the combat operations, and Nato aims to hand over security duties as soon as possible. Currently there are 68,000 soldiers in the army, and the total is to rise fast, doubling to 134,000 by 2012. In addition, the Afghan National Police Force, set up in 2001 but criticised from the start for corruption, will add some 23,000 to its present total of 76,000.

The extra cost will be about $1 billion a year, a sum far beyond the means of the struggling Government in Kabul. The West's money, troops and backing will be needed for a long time, as the Prime Minister will reaffirm to Parliament today. What is at stake is not only the viability of Afghanistan, whose collapse would boost terrorism, drug production and extremism in the heart of Asia; it is the credibility of Nato itself. And until the alliance can agree common rules of engagement, its military potential will be wasted and its political coherence undermined. Nato and the incoming US administration also need to understand their political priorities: as long as relations with Russia, Iran and other neighbouring countries are bad, there is little chance of isolating the Taleban, even if support from neighbouring Pakistan can be cut off.

Vitiating all attempts to pull Afghanistan out of the morass, however, is corruption in the Army, in daily life and in Government. President Karzai today pays generous tribute in The Times to Britain's military and political support. His thanks are timely and his stand against religious extremism welcome. More welcome still would be a concerted drive against corruption, starting with his own ministers and relations and going down to the conscript. Britain is prepared to stay the course in Afghanistan. In the run-up to next year's elections, Mr Karzai needs to show that he can deliver clean, effective government for all his compatriots.