US Secretary of State Condoleezza Rice, in London for talks with Gordon Brown and Foreign Secretary David Miliband, acknowledged that the alliance is facing a "real test".

With the crucial Nato summit looming in Bucharest in April, both Ms Rice and Mr Brown emphasised the need for greater "burden-sharing" within the alliance.

Their comments reflected the intense frustration in both London and Washington at the unwillingness of countries such as Germany, Italy and Spain to deploy troops in southern Afghanistan where the fighting is fiercest.

"It is true, and we've made no secret about it, that there are certain allies that are in much more dangerous parts of the country, and we believe very strongly that there ought to be a sharing of that burden through the alliance," Ms Rice told reporters.

Her comments were echoed in the Commons by Mr Brown, who told MPs: "What we are looking for, particularly when it comes to the Nato summit a few weeks from now, is a determination on the part of all our allies to ensure the burden sharing in Afghanistan is fair".

However, the Prime Minister was himself accused by Tory MP Patrick Mercer, a former Army officer, of ignoring a request by British commanders in Afghanistan for an additional 600 troops - a claim denied by the Ministry of Defence.

The claim came as Defence Secretary Des Browne announced that 16 Air Assault Brigade - including 2 and 3 Para and the Pathfinder Platoon - will be deployed as the lead British formation from April in the latest six-monthly troop rotation.

The Tories released figures showing that the main units which made up the brigade are some 440 troops under strength, while there are severe shortages of helicopters and helicopter crews.

The MoD insisted that it is normal for battalions on operations to be "bolstered" by reinforcements from other units. A spokeswoman said: "No unit is deployed on operations without the right number of people for the task."