Britain is keeping all options open in the fight against Islamic State (IS) as world leaders meeting in Paris agreed to provide military aid to fight the extremist network.
UK Foreign Secretary Philip Hammond said Britain had yet to decide whether to launch airstrikes against IS targets, adding that the SAS had not been sent in to rescue British hostage Alan Henning because it was not clear exactly where he was being held.
Aid worker Mr Henning appeared at the end of an IS video released on Saturday in which fellow UK hostage David Haines was killed, with a threat that he would be next.
Speaking to Sky News Mr Hammond said he understood Mr Henning's family was "going through hell," and that the government was doing everything possible to protect him.
"We have considered every possible option to support these kidnap victims, both British and others," he said.
"If we knew where they were, it would be a different story but we do not.
"We have to do what we can to protect the individual in question, and we also cannot be deterred from our strategic objective of crushing IS and the barbarous ideology it is trying to impose on the region."
Monday's summit in Paris, which was spearheaded by French President Francois Hollande and Iraqi President Fuad Masum - brought together 30 countries to co-ordinate a response to the IS threat.
The nations agreed to "support the Iraqi government by any means necessary - including military assistance".
Mr Hollande opened the summit, warning: "The terrorist threat is global and the response must be global. The cowardly murder of David Haines is a terrifying example of what is going on... There is no time to lose."
Some 930 French citizens or residents, including at least 60 women, are actively engaged in jihad in Iraq and Syria, or are planning to go there.
Mr Masum said there was a need for a "quick response" to the Islamist group which he said had "committed massacres and genocidal crimes".
Representatives of the six-nation Gulf Co-operation Council (Bahrain, Kuwait, Oman, Qatar, Saudi Arabia and the United Arab Emirates) and US Secretary of State John Kerry were also among the delegates at the conference.
However, Iran said it had rejected a request from the US to join the fight against IS because of Washington's "unclean intentions".
Sky's Europe Correspondent Robert Nisbet, in Paris, said: "This is about building a much broader alliance with regional actors, especially countries with Sunni majorities.
"This is now the pressing international issue and America would like to see all countries uniting against Islamic State."
Ahead of the talks Australia said it would send aircraft and personnel and France announced it would begin reconnaissance missions over Iraq.
Video of the British aid worker's death followed the beheadings of two American journalists, James Foley and Steven Sotloff.
Mr Cameron vowed to "hunt down" the "monsters" who killed Mr Haines , and said the crime would "strengthen our resolve" to smash the extremist network which has seized swathes of Iraq and Syria.