Britain is coming under increasing pressure to send more troops to Afghanistan as part of an American-led “quiet surge” designed to counter the worsening insurgency.

Robert Gates, the US Secretary of Defence, said yesterday that Washington was planning to send two more brigades, or about 7,000 troops, to Afghanistan next year in addition to the brigade already diverted from Iraq, and a fourth brigade in 2010. But he added that he expected Britain to match Washington’s commitment by enlarging its troop presence in Afghanistan next year as both countries reduce their numbers in Iraq.

“My understanding is that the UK may increase the size of its force,” he told journalists before the Nato meeting of defence ministers in London yesterday. “Truthfully, I don’t know if a final decision has been made.”

The Ministry of Defence denied that a further troop commitment was being considered beyond the current changeover between departing and deploying troops that will take the British contingent from 7,800 to 8,030.

The office of Des Browne, the Defence Secretary, sought clarification from the US and was told that Mr Gates was referring to an increase already announced in June. The subject of future British troop levels had not been raised at a lunch attended by both men yesterday, according to one of Mr Browne’s aides.

A defence source said, however, that Mr Gates’s comments were intended to “apply a bit of heat” to Britain to commit more troops to Afghanistan as the conflict there became the focal point in the war on terrorism. Mr Gates used a similar tactic before a Nato summit in Bucharest in April, sending a sharp letter to his German counterpart demanding that Germany shoulder more of the combat burden.

Britain, having already doubled its commitment when it took control of Helmand in 2006, was deemed to have done its duty. But with Britain’s presence in Iraq due to fall from 4,000 to only a few hundred next year, Downing Street will find it harder to argue that British troops are overstretched.

The renewed pressure from Washington comes amid growing concerns over setbacks in Afghanistan, as described by Admiral Michael Mullen, the top US military commander, in Congress last week. He told the Armed Services Committee: “I’m not sure we are winning it in Afghanistan” but added he was “convinced we can” as long as there was stategy rethink.

Mr Gates said that “the safe havens that al-Qaeda have in western Pakistan are a real concern for the US” and conceded that “while finishing the job in Iraq is important” the threat posed by the tribal areas had become the greater focus. Last month President Bush approved a secret order clearing the way for US special forces to conduct ground assaults inside Pakistan without the Government’s permission. Mr Browne has ruled out British special forces carrying out cross-border missions.

However, tensions between the US and Pakistan over civilian deaths are threatening their shaky antiterrorism alliance. Shah Mehmood Qureshi, the Foreign Minister, said that the US had failed to warn him before an attack that killed seven people on Wednesday. The missile attack was apparently aimed at a Taleban ammunition dump but residents of Angoradda, a border village, said a missile hit two houses with women and children inside.

The UN reported on Wednesday that civilian deaths as a result of coalition action had risen 40 per cent this year. In August 330 civilians were killed, the worst toll since the US-led invasion in 2001.

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