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UK Foreign Secretary William Hague said troops would not be withdrawn from the country until their "very difficult" work was complete.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......... continued