Fears are growing for a British peace activist kidnapped in Iraq after a fellow hostage was found dead.
Last week a videotape was broadcast showing Mr Kember
The body of US citizen Tom Fox has been found in Baghdad, more than three months after he was seized with Briton Norman Kember and two Canadians.
Chris Cole, a friend of Mr Kember, 74, from Pinner, north London, said he was "extremely worried" for him.
Another friend, Bruce Kent, said the vigils would continue, but added: "This is a very bad setback."
He said Mr Fox may have been singled out because he was from the US.
"For his family it is a brutal, dreadful thing to have done. They must have known he was doing nothing else than humanitarian work," he said.
Speaking about the plight of Mr Kember, he said: "It is agony. Mrs Kember must really be in torture.
"We will go on with our vigils and we will not give up. But this is a very bad setback. It is absolutely irrational."
Foreign Secretary Jack Straw said he was deeply saddened by the news about Mr Fox, saying it "clearly raises the anxiety about the other hostages".
And asked whether the killing reduced hopes for Mr Kember, Foreign Office Minister Kim Howells said: "I really hope not.
"They were dedicated to helping other people and we still make a plea to the kidnappers to free them immediately. It is terribly worrying."
Mr Kember, a retired professor, had headed to Iraq as a member of Canadian-based Christian Peacemaker Teams along with Mr Fox and Canadians James Loney and Harmeet Singh Sooden.
Mr Kember, Mr Loney and Mr Sooden were seen in a video broadcast last week on Arab satellite television station al-Jazeera, but Mr Fox did not appear.
The four were kidnapped on 26 November by a previously unknown group calling itself the Swords of Truth Brigade, which has demanded all Iraqi prisoners be freed.
There have been several vigils for the hostages. Mr Kember's wife Pat has made her own appeals on al-Jazeera, and Muslim groups have also made appeals.
CPT co-director Dr Doug Pritchard paid tribute to Mr Fox and renewed pleas for the safe release of the other three men.