Britain can not send special forces to rescue a UK hostage from Islamic State militants "because we don't know where he is", theforeign secretary says.
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 leaders - two days after the release of a video showing UK aid worker David Haines's death.
The militants have also 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 in Paris, including US Secretary of State John Kerry, to discuss the international response to IS, which controls large parts of northern Iraq and Syria.
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".
Earlier, France said it had begun surveillance flights over Iraq. Britain revealed in August that its aircraft had been gathering intelligence over Iraq.
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.