An international plan is under way to rescue civilians trapped by Islamic State (IS) fighters on a mountain in northern Iraq, British Prime Minister David Cameron said Wednesday.

Cameron declined to give details of the operation but said Britain would play a role, just as it had worked alongside the United States in conducting humanitarian aid drops to thousands of Yazidis and other minorities who have fled to Mount Sinjar.

"Clearly there is an absolutely desperate situation in Iraq, particularly on this mountainside," Cameron said after chairing a meeting of the government's emergency Cobra committee.

"I'm proud of the fact that British aeroplanes and British aid have been playing a role and will continue to play a role to help these people.

"But we need a plan to get these people off that mountain and get them to a place of safety.

"I can confirm that detailed plans are now being put in place and are underway and that Britain will play a role in delivering them."

Britain has sent Tornado fighter jets to Cyprus to be ready if needed to provide surveillance support for the aid effort, while two Chinook helicopters are also on standby to help with the humanitarian operation.

London also said Tuesday it would transport weapons from other states to Kurdish forces battling IS militants, although it is not providing the military equipment.

"What they want is ammunition and weapons like they have been using and that's what's being delivered to them, and Britain is playing a role in helping to make sure that happens," Cameron said.

The prime minister, who returned from holiday in Portugal a day early on Wednesday, also dismissed calls for parliament to be recalled from its summer recess to discuss the crisis.

There is growing pressure for Britain to join the US in conducting airstrikes.

"This is a humanitarian operation that Britain is involved in so I don't think it's necessary to recall parliament for that," he said.

"But of course I always keep this issue under review and were things to change then obviously that's something that could be done."

US Secretary of State John Kerry said on Wednesday that Washington was urgently assessing how to move the civilians off the mountain in northern Iraq.