Leaders meeting to discuss plans for British and US military move to support and training role by middle of 2013

David Cameron and Barack Obama will on Wednesday agree tentative plans for British and US troops to end their "lead combat role" in Afghanistan by the middle of next year.

Their meeting comes amid growing anger in Afghanistan over the killing of 16 civilians on Sunday by a US soldier, and follows the funerals in Britain of six soldiers killed by a bomb earlier this month, taking the death toll of UK troops over the 400 mark.

Amid fears among Nato commanders in Afghanistan that the troop "draw down" may be moving too rapidly, the two leaders will discuss plans for British and US troops to move from "lead combat" to a support and training role by the middle of 2013. This could involve what is being described as a "support combat role", though all Nato troops except those involved in training Afghan forces are due to be withdrawn by the end of 2014.

Cameron and Obama will hold discussions at the Oval Office on Wednesday, the most substantive part of a three-day official visit by the prime minister to the US, which begins on Tuesday. The president will signal a rekindling of the "special relationship" by welcoming Cameron to the White House with a 19-gun salute and a state dinner.

They will say the relationship is essential to world stability and British and US troops have lived up to Winston Churchill's maxim that they should "keep up" the co-operation of the second world war.

In a joint article for the Washington Post, they write: "Our troops and citizens have long shown what can be achieved when British and Americans work together, heart and hand, and why this remains an essential relationship – to our nations and the world. So like generations before us, we're going to keep it up. Because with confidence in our cause and faith in each other, we still believe that there is hardly anything we cannot do."

The White House talks on Afghanistan come amid warnings from Nato's International Security and Assistance Force (Isaf) that it would be wrong for a change in western military tactics to be accompanied by an accelerated withdrawal.

The Guardian has been told the head of Nato's mission, US General John Allen, has given "unequivocal advice" to the White House that he wants troop numbers to remain as high as possible until September of 2013, the end of that year's so-called fighting season.

The debate has been thrown into sharp relief by the killing of 16 Afghan civilians on Sunday by an American staff sergeant.

Allen and Obama expressed their condolences to relatives of the victims, who were killed in two villages near the soldier's base in Kandahar province.

Cameron described the killings as "an absolutely dreadful event" which should not change Nato's withdrawal plan.

Speaking in Downing Street, the prime minister said: "This really is an absolutely appalling thing that has taken place and, of course, it will have its impact. But we must do everything we can to make sure it doesn't in any way derail the very good work that American and British and other Isaf forces are doing in Afghanistan.

"We have a good plan. We have a plan which is about transitioning Afghanistan over to Afghan control. The most important thing is that we stick to that plan, we deliver that plan and then we can bring our troops home, having done a good job in giving Afghanistan at least a chance of stability and prosperity and growth for the future."

He will tell Obama that Britain remains in "lockstep" with the US on the plan to withdraw all combat troops from Afghanistan in 2014.

Last month, Leon Panetta, the US defence secretary, said US troops would "hopefully" just be involved in a "training, advice and assist role" by the latter half of 2013.

Britain asked for what is politely being described as an explanation and was reassured that the US remained committed to the timetable agreed by Nato leaders at a summit in Lisbon in 2010. Security in more than half of Afghanistan is led by the country's own security forces. The aim is for the rest of the country to be handed over by 2013.

By the end of 2014 there will be a small number of British forces left in Afghanistan who are there solely to train and support Afghan forces.

Obama has pledged to bring home by this September the 30,000 extra US troops that he sent to Afghanistan as part of his "surge". Britain is bringing back an extra 500 troops.

Nato knows recent atrocities have affected the political will to stay the course. But Whitehall sources said military commanders in Kabul will continue to urge the US administration and Downing Street not to wilt under pressure or be bounced into hasty decisions.

Any indication that the US might be hurrying towards the exit would inevitably lead to a chaotic disintegration of the 48-nation coalition fighting the insurgency, they said. That could lead to more bloodshed, not less.

"The joint chiefs of staff and British commanders will fight strongly for delaying taking any decisions about further draw down until later this summer," said one source. "They will argue now is the time to hold your nerve and to remember the blood, sweat and treasure that has been lost in Afghanistan over the last 10 years. If President [Hamid] Karzai tells Isaf to get out, that's another matter. But that doesn't make sense either. He must know Afghan security forces need Nato's help for now."

Foreign secretary William Hague, visiting the UN in New York, denied that the killings were an indication that the allied strategy in Afghanistan was "unravelling".

Hague told ITV News: "We have done a great deal of damage to al-Qaida with what we're doing in Afghanistan and it's very important we see it through to success."

The military believes a summit on Afghanistan, due to be held in Chicago in May, remains key to Isaf's future strategy. Commanders are anxious that Obama is saving up a voter-pleasing announcement as he enters the last six months before the presidential election in November.

One crucial unresolved issue in Chicago will be the level of western support given to maintaining the Afghan security forces, which will soon be 350,000 strong. "The ANSF will top out at 350,000, but the west will not fund that number beyond 2014, it is not sustainable, and it may not be necessary" said a source. "What number it comes down to will be very important for the security of the country. The Afghan economy cannot support the ANSF at the moment. The ANSF needs to be able to contain the insurgency, and it needs to be able to defend Afghanistans borders. If it comes down too much, this cannot be achieved."