The European public needs convincing that Nato's mission in Afghanistan is part of a wider fight against global terror, the US defence secretary says.
Nato leads the international forces in Afghanistan
Robert Gates warned that the future of Nato was at risk if it became a "two-tiered alliance" of countries which fought, and those that did not.
Mr Gates was speaking on the last day of a security conference in Munich.
The summit is also set to consider a threatened diplomatic crisis with Russia over Kosovan independence plans.
'Threat to alliance'
Mr Gates said it was incumbent upon Nato leaders to "recapitulate to the people of Europe the importance of the Afghanistan mission and its relationship to the wider terrorist threat".
"On a conceptual level, I believe it falls squarely within the traditional bounds of the alliance's core purpose: to defend the security interests and values of the trans-Atlantic community," he told the gathering of the world's top defence officials.
"We must not - we cannot - become a two-tiered alliance of those who are willing to fight and those who are not," he added.
"Such a development, with all its implications for collective security, would effectively destroy the alliance."
The former leader of the UK Liberal Democrat Party Paddy Ashdown - who Kabul rejected as a UN envoy to Afghanistan - said in a BBC interview on Sunday that the situation in Afghanistan was dire.
"I think Afghanistan is a failed state, I don't think it's a question of it being on the edge of it," Lord Ashdown said.
"Now there is still a majority in Afghanistan who want the international troops to stay there.
"That majority is sliding and once that graph begins to dip it is very, very difficult to turn it around."
But a senior British diplomat defended Nato's operation in Afghanistan, saying the overall strategy was working despite some problems on the ground.
Stewart Eldon, the UK's permanent representative to Nato, told the BBC it was a "mistake" to say Afghanistan would make or break Nato.
'State of emergency'
The BBC's Jonathan Marcus, at the conference, says the issue of Kosovo was also likely to make waves on the final day of the talks.
Russia's first deputy prime minister Sergei Ivanov was addressing delegates on Sunday.
Ahead of his speech, a Russian spokesman told the BBC that a declaration of independence by Kosovo and its subsequent recognition by the United States and many European Union countries would create an international state of emergency.
That, the spokesman said, could jeopardise the whole standing of the United Nations.