Britain announced a rotation of its troops in Afghanistan but said their numbers - around 7,700 - would remain about the same. Brown said Britain planned to send new helicopters and other equipment in the next few months.
Canada's minority government plans a parliamentary vote of confidence late next month on prolonging its military mission in Afghanistan, officials said on Wednesday.
The country's three opposition parties - which between them control Parliament - reject the idea of an extension. Prime Minister Stephen Harper 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.
"My view is you can't have some allies whose sons and daughters die in combat and other allies who are shielded from that kind of a sacrifice," Gates told the House of Representatives Armed Services Committee.
"Just realistically speaking, as we heard from the Canadian government just in the last couple of weeks, the willingness of those who have engaged in combat will disappear."
U.S. officials have criticized Germany for its unwillingness to send trainers into Afghanistan's restive south. Under its parliamentary mandate Germany can send only 3,500 soldiers to the less dangerous north as part of the 42,000-strong NATO mission.
Berlin has rejected mounting pressure to put its troops in the south and said on Wednesday it would send additional forces only to the north.
Gates, who will attend a NATO defense ministers meeting in Lithuania this week followed by a security conference in Munich, said he would again press NATO members on the issue. "I ... once again will become a nag on the issue," he said.
Rice's London visit was partly to smooth ruffled feathers over a recent remark by Gates in which he questioned the preparedness of some NATO members for counter-insurgency in southern Afghanistan.
The United States has 29,000 troops in Afghanistan, about half attached to NATO's 40,000-strong force. Washington plans to send another 3,200 Marines to the war zone in March and April.
NATO's top commander in Afghanistan said his force would be "minimalist" even if he received more troops. "There's no question that it's an under-resourced force," U.S. Army Gen. Dan McNeill told reporters at the Pentagon.
Under U.S. counter-insurgency doctrine, McNeill said, there should be some 400,000 security personnel - foreign and Afghan - to fight the Taliban and other insurgents.
McNeill said he did not expect NATO to provide anything like the 400,000 figure but said the West had to step up efforts to train Afghan forces, especially the police.
