BRITISH special forces are unable to mount a rescue attempt to free the hostages held by Islamic State militants because they do not know where they are being detained, Foreign Secretary Philip Hammond has admitted.

Mr Hammond said yesterday at a meeting with foreign ministers in Paris that the UK Government would have been ready to consider "all sorts of options" if they knew where the hostages were.

It follows the murder of Perth-raised aid worker David Haines, 44, by the extremist group led by a British-born killer. It followed last month's murders of two American journalists and a further threat to kill another hostage, British aid worker Alan Henning, from Manchester.

Mr Hammond said: "We don't know where he is. Obviously, if we knew where he was, we would be able to look at all sorts of options but we don't know where he is.

"We have considered every possible option to support these kidnap victims - both British and others - and if we knew where they were, it would be a different story but we don't know where they are."

Mr Hammond said Mr Henning's family was "going through hell" but that they understood there was a limit to what the Government could do to help him.

"It is a terrible time for them. We are doing everything that we can to protect him," he said.

"They understand, because we have explained to them in detail, the limitations of our abilities. We are dealing with a very barbaric organisation whose values are completely different from ours."

Foreign ministers from more than 30 countries attended the talks in Paris intended to build support for international action against IS - also referred to as Isis of Isil (the Islamic State of Iraq and the Levant) - which has taken large swathes of territory in Syria and Iraq.

Mr Hammond said he had made clear that Britain would play a "leading role" in the gathering coalition, although he would not be drawn on whether the Government was ready to join US air strikes on IS in Syria.

"I have said already in Parliament that would be an order of magnitude more difficult than air strikes in Iraq for all sorts of reasons - military, legal and technical - but we haven't ruled it out," he said.

"We haven't made a decision yet about how we will best contribute to the coalition effort against Isil. But I have said this morning in the meeting that Britain is clear that it will play a leading role in this coalition."

French president Francois Hollande said the need for action to tackle IS was urgent.

"The terrorist threat is global and the response must be global. There is no time to lose," he said.

Iraqi president Fouad Massoum called for the continuation of air strikes against IS militants in the north of his country.

"We are asking for airborne operations to be continued regularly against terrorist sites. We must not allow them to set up sanctuaries. We must pursue them wherever they are," he said.

US secretary of state John Kerry has urged allies - especially Middle East and Gulf states - to show a united front and a US official said several Arab countries had offered to join air strikes.