New video footage has emerged in which British hostage Norman Kember calls for UK troops to be pulled out of Iraq.
"I ask Mr Blair to... leave the Iraqi people to come to their own decisions on their government," says Mr Kember, 74, and from Pinner, London.
The peace activist and three other Westerners seized in Baghdad last month were shown in handcuffs and shackles.
The kidnappers have threatened to kill them if Iraqi detainees in the country are not freed by Thursday.
Mr Kember, who had travelled to Iraq with international peace group, the Christian Peacemaker Team (CPT), was kidnapped in Baghdad on 26 November.
He is being held with Canadians James Loney, 41, and Harmeet Singh Sooden, 32, and American Tom Fox, 54.
A group calling itself the Swords of Truth said it had taken the hostages and accused them of spying for foreign forces in Iraq.
Fellow hostage Mr Fox is also heard to speak on the video, a shorter version of which emerged last week.
He says: "As a representative of Christian Peacemaker Teams, we feel that continued American and British occupation is not in the best interest of the Iraqi people."
BBC World Affairs Editor John Simpson said Mr Kember had appeared to reflect his own opinions in the video as he was "very much critical" of the British and US presence in the country.
"The ludicrous thing really is that they've captured four people, of whom he's one, who are their best friends in the West," he said of the kidnappers.
Earlier on Tuesday, CPT appealed to the kidnappers to release the hostages unharmed.
It said in a statement: "While we believe the action of kidnapping is wrong, we do not condemn you as people.
"We recognise the humanity in each person, and respect it very much. This includes you, our colleagues and all people.
"We believe there needs to be a force that counters all the resentment, the fear, the intimidation felt by the Iraqi people.
"We are trying to be that force, to speak for justice, to advocate for the human rights of Iraqis, to look at an Iraqi face and say 'my brother, my sister'."
It also said that it condemned the UK and US governments for their actions in Iraq.
"We appeal to your humanity to show mercy on our brothers and let them come back safely to us to continue to work."
The UK government has said it stands ready to hear what the kidnappers have to say.
Anti-war envoy Anas Altikriti, an Iraqi, has been trying to secure the release of the four hostages.
He was sent to Baghdad by UK peace and Muslim groups and has been meeting different groups and speaking to the local media.
Muslim scholars and activists from around the world, including leaders of militant Hamas and Hezbollah groups, have also appealed for the release of the hostages.