David Cameron and Barack Obama have confirmed that British and American forces in Afghanistan will move to a "support role" from next year.
The US President, speaking a joint press conference at the White House rose garden on Wednesday, said there would be no "steep cliff" at the end of 2014 off which troop numbers would fall.
"If we maintain a steady responsible transition process which is what we have designed, then I am confident we can put Afghans in a position where they can deal with their own security," he said.
"I don't anticipate we will be making any sudden additional changes to the plan we have.
Obama also said the gradual draw down of troops would mean there would still be a "robust coalition presence" in Afghanistan during this "fighting season" to ensure the Taliban did not make quick gains.
Cameron said that the recent deaths of British soldiers in the war and the murder of civilians by an American soldier reminded everyone how "difficult" the task was.
But he said the UK and US would "not give up on this mission".
"We wont build a perfect Afghanistan... we can help ensure that Afghanistan is capable of delivering its own security," he said.
The leaders' comments came as news broke that a vehicle had driven onto the runway of Camp Bastion, the main British base in Afghanistan, then burst into flames, coinciding with the arrival as the US Defence Secretary.
Leon Panetta arrived to meet with troops, commanders and Afghan government officials just days after a US soldier went on a deadly shooting spree.
Cameron went on: "We are now in the final phases of our military mission."
"That transition to Afghan control, as agreed at Lisbon, is now well under way. And next year, as the president said, in 2013, this includes shifting to a support role.
"This is in advance of Afghans taking full responsibility for security in 2014."
In a press conference in bright sunshine outside the White House, the leaders paid tribute to relations between Britain and the US.
"There are some countries whose alliance is a matter of convenience, but ours is a matter of conviction," the prime minister said.
President Obama in turn described Cameron as an "outstanding ally, partner and friend".