The United States is to deploy up to 30,000 more troops in Afghanistan by next summer. The figure - a third more than had been anticipated - was announced by the chairman of the US joint chiefs of staff, Admiral Mike Mullen, and represents the highest figure mentioned publicly by a senior American official.

The confirmation of the scale of the troop surge - along with the increase in the upper limit of envisioned new forces - reflects concern over the course of the war in Afghanistan, which has been hit by numerous setbacks as security across the country has deteriorated in the face of Taliban advances.

In recent weeks Taliban attacks have severely disrupted the main supply line for US and Nato troops into the country from Pakistan, a route that carries up to 75% of supplies to foreign forces.

Washington is already sending some 3,000 extra troops to Afghanistan in January and another 2,800 by spring. However, previously officials have said that the number would be increased by 20,000 in the next 12 to 18 months, once approved by the new administration of President Barack Obama.

The increased scale of the commitment of US combat troops is bound to bring renewed pressure on the UK and other Nato partners to commit more troops: senior US military figures and officials have complained that America is shouldering too much of the burden in the war against the Taliban.

Last week prime minister Gordon Brown announced an extra 300 British troops for Helmand province after visiting troops in Afghanistan. Senior Downing Street sources then played down the prospect of further reinforcements this spring. However, Brown will be under pressure not to jeopardise a fledgling relationship with Obama if the new US president requests more British troops.

"Some 20 to 30,000 is the window of overall increase from where we are right now. I don't have an exact number," Mullen told reporters. "We've agreed on the requirement and so it's clear to me that we're going to fill that requirement. So, it's not a matter of if, but when," he said.

'We're looking to get them here in the spring, but certainly by the beginning of summer at the latest."

Half of the new US troops are expected to be deployed to southern Afghanistan, where British forces are stationed.

US Army General David McKiernan has asked for the extra troops to halt a growing Taliban insurgency, particularly in the east and south of Afghanistan.

Obama has pledged a renewed focus on Afghanistan, where US-led forces toppled the Taliban government in 2001 after the 11 September terror attacks. The United States currently has some 31,000 troops in Afghanistan, with some operating independently and others operating as part of a 51,000-strong Nato-led security assistance force.