The withdrawal of British troops from Afghanistan could start as early as next year, David Cameron has said.
US president Barack Obama has talked about beginning the pull-out of American soldiers from July 2011.
The prime minister told the BBC the same could be expected of UK forces "based on conditions on the ground".
But former head of the Army Gen Sir Mike Jackson said he was "wary" about setting dates, and a plan to transfer power "does not equal reality".
Shadow international development secretary Douglas Alexander told the BBC the British government had got itself into "a terrible muddle of mixed messages".
Mr Cameron was speaking in Washington, where he discussed the Afghan war with President Obama.
A major conference in Afghanistan on Tuesday backed a plan that would see Afghan forces lead security operations across the country by 2014.
In an exclusive interview with BBC Radio 5 live's chief political correspondent John Pienaar, Mr Cameron was asked whether people could expect British forces to follow the Americans in starting to pull out of Afghanistan from next year.
The prime minister said: "Yes we can, but it should be based on the conditions on the ground. The faster we can transition districts and provinces to Afghan control, clearly the faster that some forces can be brought home.
"I don't want to raise expectations about that because that transition should be based on how well the security situation is progressing.
He added: "What I have said is, people in Britain should understand we're not going to be there in five years' time, in 2015, with combat troops or large numbers because I think it's important to give people an end date by which we won't be continuing in that way.
"But I hope that with the strategy that we have - the build-up of the Afghan army, the transitioning of districts and provinces, as the president said - it will be possible to bring some troops home."
Taking his first Prime Minister's Questions in the House of Commons on Wednesday, Deputy Prime Minister Nick Clegg said the removal of British troops in a combat role by 2015 was "consistent" with the timetable for Afghan forces assuming responsibility for security.
"No timetable can be chiselled in stone but we are absolutely determined - given how long we have been in Afghanistan, given that we are six months into an 18-month military strategy, embarking on a new political strategy - that we must be out in a combat role by 2015," he said.
Former Army chief Sir Mike refused to say whether he felt Mr Cameron's 2015 target was achievable, but he acknowledged that it was "ambitious".
"The important thing for me is that a plan does not equal reality. If the conditions are not going the way of the plan, we need to keep that in mind," he told BBC Radio 4's Today programme.
"There could be an element of hostage to fortune in being too pedantic about the date. I have always been wary about dates. We seek a set of conditions on the ground."
Sir Mike said it was "always a considerable concern" that setting a date would simply encourage the Taliban to hold out until then to resurge.
He said that power could only be transferred to Afghans when they were able to maintain their own security, adding: "I don't think anybody would contemplate ceasing international military operations unless that very clear condition has been met."
BBC deputy political editor James Landale, who is travelling with the prime minister, says Mr Cameron has already set deadlines of 2014 for Afghan forces to take charge of security and 2015 for the withdrawal of combat troops.
Now, says our correspondent, he has gone further by giving a possible date for British forces to start coming home.
Labour's Mr Alexander told the BBC the government was sending "mixed messages" on an issue "that was of vital importance to service personnel's families here, but also the message that we send to the Taliban".
"When you go to Afghanistan, they tell you that the Taliban say 'our enemies have watches, but we have time'.
"And if we are sending a signal that regardless of any conditions on the ground, there will be a withdrawal of British troops from a combat role, that does represent a change of policy from what was previously the position, which was to say that we should have a conditions-based approach," he said.
But in a later interview with James Landale, Mr Cameron said the possible pull-out was "no change in policy at all".
He said it was too early to start talking about numbers, but the situation would be discussed at the Nato summit in Lisbon in November.