Munich: US Defence Secretary Robert Gates on Sunday sought to convince a sceptical European public of the threat to their security posed by extremists based in Afghanistan, and warned that a continued failure of some Nato nations to send more combat troops threatened the existence of the Atlantic alliance.

"We must not - we cannot - become a two-tiered alliance of those who are willing to fight and those who are not," he told a gathering of the world's top defence officials. "Such a development, with all its implications for collective security, would effectively destroy the alliance."

Gates's speech to the Munich conference wrapped up a week of US efforts to persuade key European allies such as Germany, France, Italy, Spain and Turkey to send more combat troops to fight the resurgent Taliban in southern Afghanistan.

He issued a strong appeal to the people of Europe, saying a failure to defeat the extremists on the Afghan-Pakistan border would pose a direct terrorist threat to their cities.

"Many Europeans question the relevance of our actions and doubt whether the mission is worth the lives of their sons and daughters. As a result, many want to remove their troops," he said. "So now I would like to add my voice to those of many allied leaders and speak directly to the people of Europe: The threat posed by violent Islamic extremism is real - and it is not going away."

Faced with widespread public opposition to the fighting in Afghanistan, governments in Germany, Italy, Spain and Turkey have repeatedly resisted US pressure to send combat troops to the Taliban's southern heartland.