Before heading for talks with Gordon Brown and David Miliband, Condoleezza Rice acknowledged that Nato is facing a "real test" in Afghanistan against a resurgent Taliban.
She said the forces were engaged in "a different fight than the one Nato was structured to do", and conceded that "it has taken some time".
Most of the fighting against the Taliban is being shouldered by the US, Britain, Canada and the Netherlands.
Canada's minority Conservative government said on Wednesday that it will introduce a parliamentary motion this week to prolong the military mission beyond February 2009.
The Nato-led International Security Assistance Force has 43,250 troops in Afghanistan. The US has a further 14,000-plus troops outside the Nato force.
A country-wise breakdown:
US: 15,000 troops (mainly east)
UK: 7,800 (mainly south)
Germany: 3,210 (north)
Italy: 2,880 (west and Kabul)
Canada: 2,500 (south)
The Netherlands: 1,650 (south)
France: 1,515 (Kabul)
Poland: 1,100 (mobile)
Australia: 1,070 (south)
Other countries: Denmark (780); Spain (740); Turkey (675); Romania (535)
Stephen Harper, the prime minister, wants the 2,500 Canadian soldiers in southern Afghanistan to stay longer but says he will pull them out on schedule early next year unless Nato sends in 1,000 extra troops.
Debate is set to start next week and a vote on the motion would take place in late March if Ottawa believed Nato allies were ready to provide additional military help, they said.
The London meeting of US and British officials comes a day before Nato defence ministers begin a two-day conference in Vilnius, the capital of Lithuania.
The aim of the Vilnius summit is to try to bridge a split between nations with soldiers in southern and eastern Afghanistan and those refusing to commit forces to the frontline.
Robert Gates, the US defence secretary, has urged the participants to send forces to replace 3,200 US marines being sent as a temporary reinforcement to the Nato mission from April to October - the Taliban's traditional fighting season.
There are tentative signs that the repeated US appeals are having some effect, starting with the modest German force addition announced on Wednesday.
The rapid reaction force will be deployed in the summer to replace a Norwegian contingent, according to Franz Josef Jung, the German defence minister.
|Rice held talks with Miliband in|
London on Wednesday [AFP]
He said the force will be stationed at Mazar-i-Sharif who will provide back-up to other contingents, mainly in northern Afghanistan but also elsewhere in case of emergency.
Belgium said last week it would send four F-16 fighter jets to join the Nato force in Kandahar, its first deployment to the south.
Meanwhile, French news reports suggest that Paris is considering the dispatch of 700 paratroopers to southern Afghanistan.
A paratrooper-unit deployment would follow an earlier decision to send ground-attack French aircraft to Kandahar and embed combat training units with Afghan forces in the neighbouring southern province of Uruzgan.
The Le Monde daily said on Wednesday that Nicolas Sarkozy, the French president, could announce the deployment at a Nato summit in Bucharest, Romania, in April, although no decision has yet been taken.
The Bucharest summit is also due to adopt a "vision statement" on Afghanistan designed to boost public support for the military mission, particularly in Germany, Italy and Spain, where support has flagged.