With German opinion polls showing a sharp rise in support for a party that opposes the Nato operation, Mr Rasmussen will say in a speech in the US he fears that the public debate on Afghanistan “has started to go in the wrong direction”.

But while allegations of fraud during the Afghan presidential elections have been “disturbing”, he will add: “We must stay in Afghanistan as long as necessary and we will stay as long as necessary. Let no one think that a run for the exits is an option. It is not.”

Mr Rasmussen’s comments come as the Left party, which is opposed to the conflict, received a sharp increase in support before Germany’s general election this month.

Support for the Left, the only mainstream party that is demanding an immediate withdrawal of Germany’s 4,200 soldiers in Afghanistan, rose four points to 14 per cent in the latest poll by Forsa for Stern magazine.

It also comes in advance of scheduled moves by the Netherlands and Canada to reduce or totally withdraw forces from the country next year and in 2011 respectively. US officials are currently in talks with The Hague to keep some 500 of the about 1,600-strong Dutch force in Uruzgan province.

Mr Rasmussen fully backed calls by Britain, France and Germany for a UN conference on Afghanistan to be held this year, to focus on transition in civilian sectors. Gordon Brown, UK prime minister, signed a joint letter with Angela Merkel, German chancellor, and Nicolas Sarkozy, French president, asking Ban Ki-Moon, the UN secretary-general, to endorse the international conference building on the heavily disputed presidential elections in Afghanistan, which the leaders claimed marked “an important step in its democratic history”.

The UK government said the Afghanistan conference could take place before the end of the year in two phases, with one meeting in Kabul and the second in another venue outside Afghanistan. Mr Brown has offered London as a possible location.

Mr Brown is anxious that the conference is not seen as the moment when Nato maps out a detailed exit strategy from Afghanistan, in spite of political pressure from inside Germany for a clear plan for withdrawal.

The text refers to setting “new benchmarks and timelines in order to formulate a joint framework for our transition phase in Afghanistan”, including handing power, security and policing over to the Afghans.

Nato diplomats dismissed the idea that the call for a conference suggested that European union states were looking for an exit from Afghanistan.

“I don’t think there’s a split emerging in the alliance,” a Nato diplomat said. “But what is recognised is the need for an acceleration in investment in training the Afghan military and getting them to take responsibility district-by-district, running their own hospitals and running their schools. That is what the UN conference is aimed at doing.”

However, the diplomat acknowledged that opinion polls in Europe and the US were shifting against the war and this was worrying.

“There is no wobble happening yet – but we can see public opinion polls that are not encouraging and we need to show people that this investment is actually getting somewhere,” the diplomat said. “If we don’t do that, then support can wither away and that would be damaging for the mission.”

Nato faced fresh Afghan anger about civilian deaths on Wednesday after special forces staged a commando raid to rescue a New York Times journalist – which led to the death of his Afghan colleague.

Sultan Munadi was killed during an exchange of fire between Nato troops and gunmen holding him with fellow journalist Stephen Farrell, a dual British-Irish citizen.

**Additional reporting by George Parker