Militants have threatened to kill three Western hostages unless all female Iraqi prisoners are set free, according to a video shown by al-Jazeera TV.
The three men, who were kidnapped in Baghdad this week, appear blindfolded on the video with a masked gunman.
The station said they were being held by followers of Abu Musab al-Zarqawi.
The group says the US authorities have 48 hours to release Iraqi women prisoners held in two Iraqi prisons or it will kill the hostages.
It is not clear when the video was made or how it came into al-Jazeera's possession.
The three men - Briton Kenneth Bigley, and his US colleagues Jack Hensley and Eugene Armstrong - were abducted at dawn on Thursday from a house they shared in the capital.
In the video shown by the Qatar-based satellite TV channel , the hostages, sitting in front of a masked gunman, appear to be giving details of who they are while their abductors' demands are made.
They had been working as engineers for a building contracting firm based in the United Arab Emirates.
All three men's families have already appealed for their safe release.
The US embassy said it had a team assigned to the case and the UK Foreign Office said all efforts were being made to find Mr Bigley.
Fighters waging a 17-month insurgency in Iraq have kidnapped more than 100 foreigners in an effort to destabilise the interim government and drive foreigners from the country.
The three men's kidnapping is the latest in a string of high-profile abductions of Westerners in Iraq.
Two French journalists were abducted almost a month ago and two female Italian aid workers, along with two of their Iraqi colleagues, were grabbed from their offices on 7 September.
Watch the video.
The group said to be holding the hostages, Tawheed Wal-Jihad (Unification and Holy War), wants Iraqi women in prisons at Abu Ghraib, near Baghdad, and Umm Qasr, in the south of the country, to be released.
The US government has said it holds only two women in Iraq - high-level detainees who are understood to have been part of Saddam Hussein's regime.
One of them is Rihab Rashid Taha - the woman dubbed Dr Germ for her role in developing Iraqi biological weapons, says the BBC's Caroline Hawley in Baghdad.
The video broadcast on Saturday is the first indication of who might have abducted the three men.
Tawheed Wal-Jihad is led by Abu Musab Zarqawi, an ally of al-Qaeda who the Americans say is behind much of the violence in Iraq.
The US has offered a $25m reward for his capture.
The group has claimed responsibility for many attacks in Iraq, including the kidnapping of foreign workers and suicide bombings that have killed Iraqi officials.
Story from BBC NEWS:
Published: 2004/09/18 12:04:33 GMT
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