• Foreign Secretary David Miliband's address is beamed into the Edinburgh International Conference Centre for Nato's Parliamentary Assembly. Photograph: PA
The signal came as Slovakia yesterday said it would double its forces in the country.
At the Nato Parliamentary Assembly in Edinburgh, delegates were also told by the organisation's supreme commander that the "war can be won" if the allies are willing to commit themselves.
Admiral James Stavridis, Supreme Allied Commander, Europe, pointed out the 28 Nato member countries had a combined GDP of $31 trillion (18.5tn) and 3.5 million people serving in their armed forces.
"We have the capacity and capability to succeed in Afghanistan if we are willing to see it through," he told delegates in Edinburgh International Conference Centre.
The speeches were part of an effort to galvanise flagging support for the war – the bloodiest for the UK since the Falklands.
A delegate from Canada pointed out that his country's commitment will end in 2011, while there was concern raised that 70 per cent of European troops are confined to barracks and inactive.
The new secretary-general of Nato, Anders Fogh Rasmussen, made a plea for member states to get at least 50 per cent of their forces active in the near future. He said that more troops would be needed to help fight the insurgency and allow Nato to train Afghan security forces.
Explaining why it was necessary to stay in Afghanistan, he said: "To my mind it is obvious – that if we were to walk away and turn our backs on Afghanistan, al-Qaeda would be back in a flash. They would have a sanctuary from which to launch their strategy of global jihad, a strategy that is directed first and foremost against us.
"There's absolutely no reason to think otherwise and anyone who does so is not living in the real world."
His message was reiterated by a predecessor, Lord Robertson of Port Ellen – formerly Labour MP George Robertson – who said the "future of Nato's credibility is at stake in Afghanistan."
He urged other members not to rely on America alone, stating "it is only fair we all share the burden."
Mr Miliband stated that the conflict was not "a war without end." But he assured delegates Britain would be "ready" to send more troops as part of an agreed strategy. He added: "I, as much as anyone else, want to bring our troops back home to safety.
"But we cannot leave a vacuum which the Taleban will quickly fill and, under their umbrella, al-Qaeda will quickly follow."
The conference did not attract the large-scale protests expected, although six demonstrators were arrested trying to enter the venue.
Slovakia's prime minister Robert Fico said yesterday that his country would double the number of troops it has serving with Nato in Afghanistan to about 500.