The Army is preparing to increase its firepower in Afghanistan with a new “smart bomb”, The Times has learnt. The news came as the number of British dead reached 50, with two soldiers killed during a military operation on Saturday.

The soldiers, from 29 Commando Regiment Royal Artillery, were killed in a Taleban rocket attack in Sangin, in the north of Helmand province.

Days earlier, Des Browne, Defence Secretary, had announced that 1,400 extra troops would be sent to bolster the Nato force in the country. The deployment will increase the number of British servicemen and women to 7,700, mostly in Helmand province.

Meanwhile, in the east of Afghanistan, on the main road between the Afghan capital of Kabul and Pakistan, insurgents carried out an attack on an American Special Forces convoy that left eight people dead.

The attack started when a suicide bomber blew up his van laden with explosives close to an American Humvee. Militants then opened fire from several locations on the troops. A statement by the US military claimed that eight civilians were killed and and one US soldier was among 34 casualties. They blamed most of the injuries on militant gunfire.

However, witnesses said that US forces shot randomly at civilian cars and pedestrians as they fled the scene of the ambush. “They opened fire on everybody, the ones inside the vehicles and the ones on foot,” Tur Gul, 38, himself shot twice in the hand, told Associated Press.

At the Jalalabad hospital others described scenes of carnage along the US convoy’s five-mile line of retreat. “When we parked our vehicle, when they passed us, they opened fire on our vehicle,” said Mohammad Ishaq, 15, who was hit in his left arm and right ear. “It was a convoy of three American Humvees. All three Humvees were firing around.”

The civilian casualties prompted angry protests in the region, with demonstrators chanting “Death to America!” and throwing stones at police.

Zemeri Bashary, the Interior Ministry spokesman, promised an investigation. “The coalition says they have proof that gunmen opened fire,” he told reporters. “But I think more of the gunfire was from the [US] side.”

The Times has learnt that the Army is preparing to deploy a new system that can launch a heavyweight guided rocket through the front door of a targeted Taleban leader’s home from a range of 70km (44 miles).

The decision to send the Army’s Guided Multiple Launch Rocket System (GMLRS), which is not even in service yet, comes after an urgent debate within the Ministry of Defence over the appropriateness of sending powerful conventional warfare weapons to fight the insurgents in Afghanistan.

Defence sources said that since British troops were deployed to southern Afghanistan nearly a year ago, army chiefs had been recommending that the combat units needed more powerful artillery to fight the Taleban. But requests to deploy the Army’s self-propelled AS90 artillery, which fires 155mm shells, had been rejected. Although other Nato countries involved in combat in Afghanistan – the US, Canada and The Netherlands – have all been deployed with 155mm artillery, the British Army was sent only with a battery of light 105mm artillery pieces, which fire small 35lb shells and are unguided. The soldiers who died yesterday were with a unit equipped with the 105mm guns.

Ministers still seem to be against sending the Army’s biggest guns, the AS90s, but after a review they approved the dispatching of the new GMLRS, which can fire 12 guided rockets at once, each guaranteed to impact within 2m of its target.

The defence sources said that until the arrival of the new rocket system, commanders in Helmand province in the south who wanted more firepower than could be provided by the light 105mm guns had to turn to the RAF Harrier GR7s or GR9s to drop 500lb or 1,000lb bombs on a target from 10,000ft.

The GMLRS fires a rocket with a 200lb warhead. It has on-board inertial guidance and is also connected to GPS. Built by Lockheed Martin, the US defence contractor, the GMLRS is being purchased for £250 million. The Army received its first batch in January, and gunners of 39 Regiment Royal Artillery are training with it before their deployment in about June.

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