The issue of burden-sharing in Afghanistan goes right to the heart of the NATO alliance, says U.S. Defense Secretary Robert Gates.

"We must not -- we cannot -- become a two-tiered alliance of those willing to fight and those who are not," he said during a speech Sunday at the Munich Conference on Security Policy.

"Such a development, with all its implications for collective security, would effectively destroy the alliance."

• MacKay optimistic about NATO support for mission

Gates also praised allies for their contributions in Afghanistan, where al Qaeda had a safe haven under the Taliban's rule. But he also said more must be done to defeat the resurgent Taliban, who were pushed from power in late 2001.

"In NATO, some allies ought not have the luxury of opting only for stability and civilian operations, forcing other allies to bear a disproportionate share of the fighting and the dying," Gates said.

He named no individual countries, but U.S. officials have been pressing Germany to do more.

Canada's Defence Minister Peter MacKay was at an informal meeting of NATO defence ministers in Lithuania to lobby for more troops to help Canada in Kandahar.

There are 2,500 Canadian troops in Kandahar. The recent Manley panel report said NATO should provide an additional 1,000-soldier battle group to assist Canada, along with equipment like medium-lift helicopters and drones.

MacKay said Saturday that progress had been made towards that request.

NATO is in charge of the Afghanistan military mission through the International Stabilization Assistance Force (ISAF).

There are about 42,000 ISAF troops, of which the U.S. (14,000) and Britain (7,800) are the biggest contributors. The U.S. has another 13,000 troops operating outside ISAF under Operation Enduring Freedom, hunting militants and training Afghan forces.

U.S. Army Gen. John Craddock, the NATO supreme commander, told reporters before Gates' speech that the fight in Afghanistan suffers from a "paucity of troops."

He said they are short they are short at least three manoeuvre battalions, as well as intelligence, surveillance and reconnaissance tools for tracking movements on the ground.

"Give us the resources," Craddock said.

Gates admitted the U.S. had learned from the mistakes it made in Iraq, such as the need to closely integrate civilian-led stabilization efforts with the military side.

He suggested that Europe doesn't take the threat of Islamic extremism very seriously -- or the role of Afghanistan in fomenting that extremism.

"The threat posed by violent Islamic extremism is real -- and it is not going to go away," he said.

"It raises the question: What would happen if the false success they proclaim became real success? If they triumphed in Iraq or Afghanistan, or managed to topple the government of Pakistan? Or a major Middle Eastern government?

"Aside from the chaos that would instantly be sown in the region, success there would beget success on many other fronts as the cancer metastasized further and more rapidly than it already has," said Gates.

Despite the so-called war on terror, a U.S. National Intelligence Estimate report last summer found that al Qaeda has rebuilt its operational capacity to a level not seen since before the Sept. 11, 2001 terror attacks.

U.S. officials claimed Saturday that top al Qaeda and Afghan Taliban leaders are currently in the tribal areas of Pakistan -- an accusation Pakistan hotly denies.

With files from The Associated Press