The Foreign Office says it is still hopeful British hostage Norman Kember will be released unharmed, after the showing of a second video on Arabic TV.
The kidnappers gave a deadline of Thursday
Al-Jazeera quoted the kidnappers saying they would kill the four men being held in Iraq, unless prisoners in Iraqi and US centres were released by 8 December.
Iraq war opponent Mr Kember, 74, of London, was seized in Baghdad.
A British anti-war campaigner is due to arrive in the Iraqi capital later to appeal directly for his release.
Anas Altikriti - who is of Iraqi origin - is being sent by the anti-war movement, representing the Muslim Association of Britain, Stop the War and CND.
In a BBC interview, the association's spokesman Itisham Hibatullah said he was hopeful Mr Altikriti would be able to influence the kidnappers.
"This is the unknown territory we are going into but we have hope once Anas started speaking to the local community leader as he has already done through Al Jazeera television last night.
"What we are trying to do is influence their minds to say that he's a true friend of Iraq and a peace campaigner," he said.
Mr Kember has been held since Saturday along with three other peace activists - American Tom Fox, 54, and Canadians James Loney, 41, and Harmeet Singh Sooden, 32.
Mr Kember and Mr Fox were shown on the latest video calling for the kidnappers' demands to be met, al-Jazeera said.
A previously unknown militant group, the Swords of Truth Brigade, have claimed the captives were undercover spies working as Christian peace activists.
Mr Kember had been working with a Canadian-based organisation, Christian Peacemaker Teams.
Friends and peace campaigners held a vigil for Mr Kember in London overnight.
David Cockburn, who knows Mr Kember through work with the group, expressed hope that he would eventually be released, despite the video.
"It's very saddening, but on the other hand it does say that they are alive - and that is good," he said.
"And also the deadline they have set next Thursday gives quite a significant time for changes to come about, so that, I think, is positive."
But the Fellowship of Reconciliation, of which Mr Kember is a trustee, said it was "extremely worried" by the new video.
Director Chris Cole said the group had hoped calls to release the hostages - focusing on them being men of peace - would have had an impact.
"The news tonight is extremely upsetting and worrying, and we repeat again that these are innocent people, they are not spies in any way.
'Redouble their efforts'
"All four of them are the last people that should have been taken.
"Everyone now has to redouble their efforts to get all four of them out, and call on everybody to do all they can to secure their freedom."
The Foreign Office said it was in touch with Mr Kember's family through a family liaison officer.
"We are also in touch with the Iraqi authorities, and of course we are hopeful that Mr Kember and his colleagues will be released unharmed and reunited with their families," said a spokesman.
The videotape shown on Friday evening apparently showed the two Canadian hostages receiving food from their captors.