Kidnappers of four Western peace activists in Iraq have threatened to kill them unless all Iraqi prisoners are released from U.S. and Iraqi prisons.

The kidnappers issued the threat in a videotape broadcast Saturday by the Arab television station al-Jazeera. The videotape, dated January 21, apparently shows the two Canadians, one Briton and one American standing against a wall. The four disappeared on November 26.

Al-Jazeera said the kidnappers issued a statement with the tape saying it was the "last chance" for U.S. and Iraqi authorities to save the hostages.

The latest development comes a day after al-Jazeera broadcast a video of two Germans kidnapped last Tuesday.

In Iraq Saturday, authorities say a policeman was killed and another wounded in a roadside bomb attack in the western city of Fallujah.

Some information for this report was provided by AFP, AP and Reuters.