BAGHDAD, Iraq - The kidnappers of four Christian peace activists threatened to kill the hostages unless all prisoners in U.S. and Iraqi detention centers are released, according to a videotape broadcast Friday by Al-Jazeera television.

The tape showed what the broadcaster said were two Canadian hostages. An American and a Briton are also being held. The kidnappers gave the two governments until Dec. 8 to meet their demands, Al-Jazeera quoted a statement delivered with the tape as saying.

The Canadians were shown eating from plates of what appeared to be Arabic sweets. In a second clip, the British and American hostages were shown to talking to the camera but no audio was transmitted. All four men appeared frightened.

The hostages called on the U.S. and British governments to withdraw from Iraq, Al-Jazeera reported, quoting a statement from the kidnappers, who earlier identified themselves as the Swords of Righteousness Brigade, a previously unknown group.

The corner of the video showed two crossed black swords and the name of the insurgent group written in red Arabic script the same emblem shown in an earlier video of the hostages.

The group has claimed that the kidnapped men, from the Chicago-based Christian Peacemaker Teams, were spies working under the cover of peace activists. The group denied spying.

Greg Rollins, a Canadian member of Christian Peacemaker Teams, said news of the death threat was disturbing.

"It"s a bit of a shock," said Rollins, who had not yet seen the new video. He said he hoped the kidnappers would respond to messages of support from Muslims and other Arabs for the group, which opposes the war in Iraq.

"Hopefully they will continue to read those things and get a positive idea of what we do here and who my teammates are."

The activists were seized Saturday near Baghdad University. A car blocked their car, gunmen got out, threw the driver and translator out and drove away with the four captives, security officials said Thursday. The officials spoke on condition of anonymity because they are not authorized to talk to the media.

The men were kidnapped at the same place where Giuliana Sgrena, a reporter for the Italian newspaper Il Manifesto, was seized Feb. 4 and held for a month by a group calling itself Mujahedeen Without Borders. That group had not been known before and has not been heard from since, but may be using a different name.

The Christian activists Tom Fox, 54, of Clear Brook, Va.; Norman Kember, 74, of London; James Loney, 41, of Toronto; and Harmeet Singh Sooden, 32, of Canada had been repeatedly warned by Iraqi and Western security officials that they were taking a grave risk by moving about Baghdad without bodyguards.

"He was in Iraq solely for peaceful purposes and was passionate about taking an active role in giving humanitarian aid to the Iraqi people. There were no other motives for him to go to Iraq," Sooden"s family said in a statement.

Kember"s family said he was opposed to the U.S. presence in Iraq.

"He was in Iraq because of human rights abuses against the Iraqi people," the family said in a statement.

Loney"s family said they, like the others, were praying for his release.

A German archaeologist, Susanne Osthoff, 43, also disappeared recently. On a video made public Nov. 29, kidnappers threatened to kill her unless Germany stops dealing with the Iraqi government. Osthoff, who speaks Arabic, had helped distribute aid in Iraq.

The new abductions of Westerners come after a monthslong hiatus. The gruesome killings of past hostages have brought criticism from many Arabs, including an influential Egyptian cleric who issued a fatwa saying that since Iraq remains in a state of war, the kidnapping of those involved in the war is allowed but hostages should not be killed.

Some security experts believe the surge of kidnappings may be a result of lax security, attempts by insurgents to mix up tactics in an intense period of car bombings and suicide attacks or a desire to disrupt this month"s elections.

Insurgents, including al-Qaida in Iraq, have seized more than 225 people in Iraq, killing at least 38.