The former leader of the Soviet Union, Mikhail Gorbachev, has warned Nato that victory in Afghanistan is "impossible".
Mr Gorbachev said that the US had no alternative but to withdraw its forces if it wanted to avoid another Vietnam.
As Soviet leader, he pulled his troops out of Afghanistan more than 20 years ago after a 10-year war.
UK Foreign Secretary William Hague said troops would not be withdrawn from the country until their "very difficult" work was complete.
In an interview with the BBC's Moscow correspondent Steve Rosenberg, Mr Gorbachev praised President Barack Obama for his decision to begin withdrawing troops next year, but said the US would struggle to get out of the situation.
"Victory is impossible in Afghanistan. Obama is right to pull the troops out. No matter how difficult it will be," Mr Gorbachev said.
He said before the Soviet Union withdrew from Afghanistan, an agreement had been reached with Iran, India, Pakistan and the US. "We had hoped America would abide by the agreement that we reached that Afghanistan should be a neutral, democratic country, that would have good relations with its neighbours and with both the US and the USSR.
"The Americans always said they supported this, but at the same time they were training militants - the same ones who today are terrorising Afghanistan and more and more of Pakistan," Mr Gorbachev said. Because of this, it would be more difficult for the US to get out of the situation.
"But what's the alternative - another Vietnam? Sending in half-a-million troops? That wouldn't work."
The best that Nato could hope to achieve, he said, was to help the country get back on its feet and rebuild itself after the war.
Meanwhile the NATO secretary-general, Anders Fogh Rasmussen, has confirmed that he is talking to Moscow about how the Russians could help with the mission in Afghanistan.
Mr Rasmussen said he hoped important decisions on co-operation "on a broad range of areas" would be made at the NATO-Russia summit in Lisbon next month, but added that Russian troops would not be sent to Afghanistan.
"For historical reasons we will not see Russian boots on the ground in Afghanistan. But Russia can contribute in other ways. They can provide helicopters, they can also conduct training of Afghan security forces in Russia, we can co-operate on counter-narcotics," he said.
In a House of Commons statement, Mr Hague said life in Afghanistan had improved in recent years, particularly in education and healthcare. However, he added: "We have not yet achieved our central objective, which is our own national security and that is why we have to continue to work at this, even though it is very difficult.
"So I will not claim to you that we have achieved swathes of our objectives - our central objective has not yet been met and we have to continue to work at it."
Democracy 'in trouble'
In his BBC interview, Mr Gorbachev also expressed fears about how the political situation was developing in Russia.
"I am very concerned, we're only half way down the road from a totalitarian regime to democracy and freedom. And the battle continues. There are still many people in our society who fear democracy and would prefer a totalitarian regime."
He said the ruling party, led by Prime Minister Vladimir Putin, "has been doing everything it can to move away from democracy, to stay in power".
Mr Putin served two terms as president, the limit on consecutive terms, before becoming prime minister in 2008.
He has hinted he may run for president again when President Medvedev's current term expires in 2012
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