David Cameron says he will not rule out air strikes against Islamic State (IS), which is holding a British hostage.

Asked if strikes could be launched without Syrian backing, he said President Assad’s government was “illegitimate” given his “war crimes”.

The prime minister told the BBC action must not be “Western intervention over the heads of neighbouring states”.

The BBC understands Tory MPs are being canvassed for their views on military action against IS by party whips.

Ransom

Extremist group IS, which has seized large swathes of territory in Iraq and Syria and declared a new caliphate – or Islamic state – has killed two US hostages in recent weeks.

It has threatened to next kill British aid worker David Haines, who it has been holding for more than a year, unless US air strikes on its positions in Iraq are halted.

David HainesDavid Haines was captured in March last year with an Italian aid worker and two Syrians

Mr Haines, a 44-year-old father of two from Perth, was taken hostage in the village of Atmeh, in the Idlib province of Syria, in March 2013.

The threat to kill the Briton came during a video posted online on Tuesday showing the killing of US journalist Steven Sotloff. IS killed another US journalist, James Foley, last month.

Speaking as Western leaders gathered in Wales for a Nato summit, the prime minister said although the UK would not pay a ransom to free Mr Haines, he was “personally supervising” efforts to release him.

Mr Cameron is to urge Nato leaders to stop paying ransoms to terrorists to secure the release of hostages.

Ahead of a dinner at Cardiff Castle, he said he would warn leaders that money paid to terrorist kidnappers funded arms, weapons and terror.

“It’s utterly self-defeating,” he said. “It’s worse than self-defeating – it’s a risk to us back home.”

In a reference to countries such as France and Italy, which are believed to pay ransoms, Mr Cameron noted that the G8 had agreed at Loch Erne last year to cease all such payments.

He said what mattered was “not the signature on a document, but not paying ransoms”.

Earlier, the prime minister also said that – if requested by the Iraqi government – air strikes in Iraq would be legal

David Cameron tells Sarah Montague about how the UK plans to face the challenge of IS

But when questioned on Radio 4’s Today programme whether the UK could legally take military action in Syria without such a request from President Assad, Mr Cameron said he believed it was possible.

“The Iraqi government is a legitimate government. We believe it is about to become more legitimate with a new prime minister with the backing of all of his country – whereas President Assad has committed war crimes on his own people and is therefore illegitimate.”

In other developments:

Although the US is carrying out air strikes against IS, Britain has not yet done so and Mr Cameron said any action in the region must be “properly thought through and patiently delivered”.

He told the BBC that the UK was helping Kurds on the ground fighting IS with armaments and that it should look to “potentially training some of their forces”.

He added that the UK should also make sure there was an Iraqi government in Baghdad that could represent “all of the country”