Arabic television station al-Jazeera has broadcast a new videotape of four Western hostages being held in Iraq.
It was not possible to hear what the men were saying
The TV quoted a statement in which the kidnappers said they would kill the men unless all prisoners in Iraqi and US centres were released by 8 December.
The peace activists - two Canadians, an American and a Briton - were accused by their captors of being spies.
The American and the Briton were shown calling for the kidnappers' demands to be met, al-Jazeera said.
The videotape, from a group calling itself Swords of Truth, apparently showed the two Canadian hostages receiving food from their captors.
Few details have emerged of how the group was abducted last week. It is thought they were in a district of western Baghdad with minimal security when they were snatched.
In a videotape shown on al-Jazeera earlier this week, the group accused the men of being spies.
The US-based Christian Peacemaker Teams (CPT) identified the men as Briton Norman Kember, 74, American Tom Fox, 54, and Canadians James Loney, 41, and Harmeet Singh Sooden, 32.
The group is one of the few remaining aid groups operating in Iraq.
A spokesman for the UK Foreign Office said it was aware of the media reports of the videotape and was looking into it.