Washington: British Prime Minister David Cameron has arrived in Washington, to back President Barack Obama's bid to cool "loose talk" over war with Iran and as Afghan war policy faces sharp scrutiny.

Cameron became the first foreign leader to join Obama on Air Force One as the two leaders headed off to watch a college basketball game in Ohio, in a gesture meant by the White House to highlight a bond between the two men.

The meat of the visit will come on Wednesday as Obama welcomes Cameron to Oval Office talks; the two leaders hold a press conference and enjoy the pageantry of a state dinner at the White House.

Cameron's arrival comes at the time of extreme stress for the NATO-led mission in Afghanistan, following a string of incidents which culminated into the massacre of civilians on Sunday by a renegade US soldier.

Both sides say they are committed to a timetable which would see the last Western combat troops withdrawn from the country at the end of 2014, but there is rising debate about the pace of the drawdown.

Obama warned on Monday against a "rush for the exits" in Afghanistan, making the case that the soldier's murderous rampage which killed 16 civilians, mostly women and children, should not throw US strategy off -course.

British Ambassador to Washington Sir Peter Westmacott signalled that Cameron would take a similar tack in his meetings with Obama.

"I don't have the impression from the responses from any of the governments, from the authorities concerned, that these terrible incidents knock the strategy of course," Westmacott told reporters.

But behind the scenes, there is a growing impression that Obama, and some of his Western allies who are facing declining public support for the war, are keen to promote a quicker drawdown than military brass might want.

The White House could reduce the US footprint in Afghanistan by an additional 20,000 troops next year.

In a joint Washington Post article, the two leaders said they would prepare the NATO summit in Chicago in May, which will include "shifting to a support role" and ensuring Afghanistan is never again a haven for Al-Qaeda.

Cameron's trip comes in the week following a crucial visit to Washington by Israeli Prime Minister Benjamin Netanyahu dominated by the possibility that Israel could mount a unilateral strike against Iran in the coming months.

"Both of our governments have made clear that we don't think that would be helpful," said Westmacott.