Prime Minister David Cameron was quoted by Sky News as saying on Sunday Syrian President Bashar al-Assad could stay on as part of a transitional government in the short term.

Sky's correspondent, travelling with Cameron to the United States, said Cameron was not ruling out that Assad could be part of a transition, but "what he is very clear about is that Assad cannot be part of Syria's future in the long run".

Earlier, the Telegraph quoted a government source as saying Cameron was open to keeping Assad in power while a unity government was formed in the country after more than four years of civil war.

The Labour Party will on Monday call for the Syrian government to be referred to the International Criminal Court for suspected war crimes. A Labour Party aide said Assad could not be part of a peace plan.

Cameron is hoping to win support from Labour to extend British military action against Islamic State militants to those in Syria as well as Iraq, but the election of anti-war campaigner Jeremy Corbyn as Labour leader this month has complicated his plans.

Speaking at Labour's annual conference on Monday, the party's foreign affairs spokesman Hilary Benn will not rule out backing air strikes, but say any action must be part of a broader plan to bring peace to Syria. He will call on Cameron to push for a United Nations Security Council Resolution on Syria.

"There's been a lot of talk about air strikes but to bring peace, stability and security to Syria we need a much broader, more comprehensive plan than just trying to deal with ISIL," Benn will say, according to extracts released in advance.

"This will require political, diplomatic and humanitarian will too."