Hammond in bid to reassure US over action

He indicated ministers would be prepared to go back to Parliament to seek permission to extend British military action into Syria if US Central Command considered it “militarily useful”.

The news comes as RAF Tornado fighter bombers have carried out fresh air strikes on Islamic State militants in Iraq.

Tornado GR4s, flying from RAF Akrotiri in Cyprus, attacked an IS stronghold near Ramadi using Brimstone missiles and Paveway IV guided bombs, the Ministry of Defence said.

US warplanes, supported by Arab allies, have been conducting air strikes in an attempt to halt an assault by IS forces on the Syrian town of Kobani, close to the border with Turkey.

A Pentagon spokesman said US and coalition air strikes have forced some Islamic State militants out of the Syrian border town of Kobani, but it may yet fall under the extremists’ control because air power alone cannot prevent it.

“Kobani could be taken. We recognise that,” Rear Admiral John Kirby, the Pentagon press secretary, told reporters.

“We’re doing everything we can from the air to try to halt the momentum of Isil against that town,” he added.

“Air power is not going to be alone enough to save that city

Mr Hammond, who held talks in Washington with US Secretary of State John Kerry about the crisis, said: “We absolutely have not ruled out playing a role in Syria.

“We will require further parliamentary approval if we decide that that is the right thing for us to do.

“We would see this as a military question - is there a militarily useful role that UK assets could play.”

Meanwhile former Middle East minister Sir Hugh Robertson yesterday defended the Government’s handling of Islamic State hostage cases but he acknowledged that “we don’t know a great deal” about what is happening inside the brutal militant organisation.

Officials and ministers have been criticised by the family of Alan Henning, who was murdered by IS, for imposing a news blackout after his capture in December last year.

But former foreign office minister said it was often necessary to buy time for negotiations in hostage cases.

He said it may be worth re-examining the policy, but there were “sound reasons” for it to remain.

Aid worker and former taxi driver Mr Henning, 47, was kidnapped in December last year by IS militants, and his brother Reg Henning claimed that it might have made a difference if the family had been allowed to speak out earlier about his capture, rather than only being able to break their silence when he first appeared in a video released online.

Meanwhile Syria has declared four previously unmentioned chemical weapons plants, the United Nations has heard.

Diplomats said Sigrid Kaag, a special representative of the UN secretary general, told the Security Council during closed consultations that three of the facilities were for research and development and one was for production, and no new chemical agents had been associated with the four sites.