The captors, called Suyouf al-Haq (Swords of the Truth) had threatened to kill the hostages on Thursday but extended the deadline until Saturday (10 December).
Footage aired by Aljazeera on Wednesday, showed unverified images of two presumed hostages whose faces were not revealed.
Four members of the Christian Peacemakers Team (CPT), a non-governmental organisation, were abducted in Baghdad on 26 November by the group which threatened to kill them unless all detainees in US and Iraqi prisons were freed.
The captives are Canadians James Loney, 41 and Harmeet Singh Sooden, 32; US national Tom Fox, 54, and Briton Norman Kember, 74.
The images, shown without sound, were of two people wearing white tunics and with their hands in chains.
A representative for the peace group, Sandra Radevez has appealed for their rlease.
Briton Norman Kember, 74, is one
of the Westerners in captivity
She said while the peace team considered abduction a crime, it does not denounce the abductors but respects their human dignity as it respects the dignity of the hostages.
British Foreign Secretary Jack Straw also renewed calls on Wednesday for the release of the Briton and his three colleagues, saying: "These four men are all campaigners for peace, dedicated to the helping of others, and we ask for their release."
"The message of this latest statement is not clear. If the kidnappers want to get in touch, we want to hear what they have to say."
Meanwhile, a Jordanian cleric jailed in Britain for links to al-Qaida called for the captives' release in a video appeal aired on Wednesday.
"I, your brother Abu Qatada ... beseech my brothers in the Swords of Truth in Iraq ... to release them in accordance with the fundamental principle of mercy of our faith"
Jailed al-Qaida-linked cleric
He said the men should not be punished for the policies of their governments.
"I, your brother Abu Qatada ... beseech my brothers in the Swords of Truth in Iraq, who are imprisoning the four Christian peace activists, to release them in accordance with the fundamental principle of mercy of our faith," he said in an appeal aired on Arab television networks on Wednesday.
"Our prophet said mercy should be shown unless there is a reason in Sharia (Islamic law) that prevents it," he added.
British authorities say Abu Qatada was a leading inspiration for al-Qaida in Europe.