Nato Secretary General Jaap de Hoop Scheffer has said the organisation cannot "run away" from Afghanistan, as members hold talks in the Netherlands.
If Nato left Afghanistan now it would become a failed state and a training ground for terrorists, he told the BBC.
US calls for its allies to send more troops to Afghanistan were high on the agenda of the meeting in Noordwijk.
Washington provides more than 15,100 of over 41,100 Nato troops in Afghanistan, where the Taleban is resurgent.
Nato's secretary general said he would float the idea at the talks of more national contingent rotations around the country, although he stressed it was a long-term idea.
Meanwhile, French officials indicated they were planning to send up to 50 military trainers to the south of the country.
The BBC's Caroline Wyatt in Noordwijk says US, UK, Canadian and Dutch troops are bearing the lion's share of fighting a revived Taleban.
National caveats currently prevent some countries - such as Germany, Italy, France and Spain - from either fighting or from being based in the more dangerous provinces.
This has led to a perception of an unfairly shared burden, says our correspondent.
Before the two days of talks between 26 Nato defence ministers opened on Wednesday at the Dutch seaside resort, Mr De Hoop Scheffer said members must fulfil their Afghan mission.
He told the BBC: "We cannot simply run away from the Afghan people and say 'thank you very much ladies and gentlemen but this is it we are leaving'.
"Secondly, are we going to see Afghanistan become a failed state again? Be a training camp for terrorists with all the consequences for London, for Amsterdam, for Brussels, and for the United States? Quite honestly I do not think so."
Before the meeting, US Defence Secretary Robert Gates had complained some European nations were not pulling their weight in Afghanistan.
A Pentagon spokesman told Reuters: "The secretary does not seek to single out or embarrass any one nation but remind this powerful alliance of their moral responsibility and collective commitments."
As talks got underway, the Netherlands - which has seen 10 of its soldiers killed in Afghanistan - also appealed for more help.
"One thing is certain, there is no such thing as a free ride to peace and security," said Dutch Defence Minister Eimert Van Middelkoop.
The Netherlands has warned its parliament may not extend the mission beyond August, unless they get more back-up.
Six years since the toppling of the Taleban, there is an air of concern, if not crisis, about the 38-nation mission to Afghanistan, say analysts.
As well as a Taleban fight back and record poppy crops, rebuilding has gone more slowly than expected, while civilian and military casualties have tested public support for the mission. ( BBC )