Britain cannot send special forces to rescue a UK hostage from Islamic State militants "because we don't know where he is", the foreign secretary has said.
Philip Hammond said if the UK and other nations knew where hostages were held "it would be a different story".
He spoke after a Paris summit of foreign ministers and two days after the release of a video showing UK aid worker David Haines's death.
The militants have threatened to kill a second Briton, Alan Henning, 47.
Mr Henning, a married father-of-two from Salford, previously worked as a taxi driver but had been a volunteer on an aid convoy in Syria before he was captured.
Mr Hammond met foreign ministers, including US Secretary of State John Kerry, to discuss the international response to IS, which is also known as Isil and Isis, and controls large parts of northern Iraq and Syria.
Asked why the UK did not deploy special forces to rescue Mr Henning, Mr Hammond said: "Because we don't know where he is, it's as simple as that.
"Obviously, if we knew where he was, we'd be able to look at all sorts of options."
He added: "We've 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."
Mr Hammond said it was "a terrible time" for the Henning family.
But he added: "They understand the limitations of our abilities and that we are dealing with a very barbaric organisation whose values are completely different to ours.
"We have to do everything we can to protect the individual in question but we cannot be deterred from our strategic objective of crushing Isil."
In a joint statement, foreign ministers from 30 countries pledged their commitment "to supporting the new Iraqi government in its fight... by any means necessary, including appropriate military assistance".
Mr Hammond said Britain would play "a leading role" in that coalition.
Opening the summit, French president Francois Hollande said IS "threatens the whole of the Middle East and the rest of the world", adding: "Every country is involved and we have to do everything to stop the indoctrination of our young, break the jihadi networks and remove the group's funding."
It follows US air strikes targeting IS militants in Iraq in recent weeks.
The UK has not been involved in the air strikes but has flown surveillance missions and donated heavy machine guns and ammunition to authorities in Iraq to help fight IS.
In the video released on Saturday night, a masked man holding a knife who appears to have a British accent is pictured beside Mr Haines and recorded as saying the aid worker had to "pay the price" for Mr Cameron's promise to arm the Kurds.
Mr Haines, a father of two, had been seized in Syria in March 2013 and was being held by IS militants.
The 44-year-old was born in Holderness, East Yorkshire, went to school in Perth and had been living in Croatia with his second wife, who is Croatian, and their four-year-old daughter. His parents live in Ayr.
Meanwhile, Mr Haines's teenage daughter, Bethany, wrote on a Facebook page dedicated to him: "I was really touched by the messages of support during this hard time," and said her father "would be really touched and grateful".
His death came after IS militants killed two US hostages in recent weeks, also posting videos on the internet.
They had threatened to kill Mr Haines during a video posted online showing the killing of US journalist Steven Sotloff earlier this month. They also released a video of the killing of US journalist James Foley last month.
Speaking on Sunday, Prime Minister David Cameron said he would take "whatever steps are necessary" to keep the UK safe, calling Mr Haines a "British hero".
He said the "menace" of IS had to be destroyed in a "calm, deliberate" way.
Downing Street has so far ruled out any immediate recall of Parliament but former military chiefs are among those who have called for the UK to join US air strikes.
The prime minister spoke to Mike Haines, the aid worker's brother, by telephone on Sunday to express his condolences to his family.
A spokesman for Mr Cameron said: "The government will not change its approach one iota in response to acts of barbarity perpetrated by Isil (IS)."
US President Barack Obama said the US would work with the UK and a "broad coalition of nations" to "bring the perpetrators of this outrageous act to justice".
Conservative MP and retired Army Colonel Bob Stewart said that Britain should not rule out sending in ground troops.
He told BBC Radio 4's Today programme: "It's feasible. It's not desirable. Personally, I think we can't say that we will never, ever put infantry on the ground, which is what we're talking about, because circumstances change."
He added: "What we want to do is build a coalition of people, local countries, who will actually go in on the ground."
Mayor of London Boris Johnson said ground troops were "off the table for the foreseeable" future and that it would be "very difficult" to persuade the British public that such a commitment was necessary.