A group led by a Jordanian terrorist announced Monday that it has killed a kidnapped American construction contractor, posting on an Islamic Web site a grisly video of the captive being beheaded.

The video identified the American as Eugene "Jack" Armstrong, one of three westerners the group abducted last week and threatened to kill unless U.S. forces in Iraq released all female Iraqi prisoners. Also held captive were another American contractor and a British engineer.

The authenticity of the video could not immediately be confirmed, but news agencies reported that a masked man who read a statement on it sounded like Abu Musab Zarqawi, a Jordanian who has been identified as the speaker on similar videos in recent months.

After reading the statement, the man took out a large knife and sawed off the head of a blindfolded man wearing an orange jumpsuit. The nine-minute video showed the banner of Zarqawi's Monotheism and Jihad group, which claimed to have kidnapped Armstrong and the two other men.

The video was posted after the expiration of a 48-hour deadline for the release of all Muslim women detained at the Abu Ghraib prison near Baghdad and in another prison in the southern port city of Umm Qasr.

U.S. officials have said the only women held by coalition forces in Iraq were biologists allegedly connected to a weapons program under former president Saddam Hussein. They said the women, Huda Salih Mahdi Ammash and Rihab Taha, are both being held at Camp Cropper near the Baghdad airport.

Armstrong was abducted Thursday in Baghdad with another U.S. contractor, Jack Hensley, and a British engineer, Kenneth Bigley, in a bold raid on their two-story house in one of the capital's most affluent neighborhoods. In an operation similar to the kidnapping of two Italian aid workers from their offices the previous week, as many as 10 gunmen in a minivan pulled up in front of the contractors' compound, barged inside the gate and snatched the three men from their house without firing a shot, neighbors said.

Armstrong grew up in Hillsdale, Mich., but left the area around 1990. His brother, Frank, still lives there. Armstrong's work in construction took him around the world; he lived in Thailand with his wife before going to Iraq.

The other American hostage, Hensley, 48, made his home in Marietta, Ga., with his wife Patty and their 13-year-old daughter. Armstrong, Hensley and Bigley, 62, all worked for Gulf Services Co. of the United Arab Emirates, a private contractor involved in reconstruction projects in Iraq, a company spokesman said.

The British government and the brother of Bigley had appealed for his and the Americans' release in statements broadcast repeatedly Monday on the Arab satellite television station al-Arabiya, the Associated Press reported.

"Ken has enjoyed working in the Arab world for the last 10 years in civil engineering and has many Arabic friends and is understanding and appreciative of the Islamic culture," said Philip Bigley.

"He wanted to help the ordinary Iraqi people and is just doing his job," he said. "At the end of the day, we just want him home safe and well, especially for my mum Lil."

The AP reported that Hensley's wife, Patty, appeared on al-Jazeera and said her husband, like all Americans in Iraq, was there to help the Iraqi people.

Patty Hensley told ABC's "Good Morning America" in an interview broadcast Monday that her husband had been optimistic about his safety in Iraq, but that had changed in recent days, when the group's Iraqi security guards stopped showing up for work or found excuses to leave.

On NBC's "Today" show Monday, Ty Hensley said his brother went to Iraq in February after failing to find construction work because he needed money to support his family. He said his brother recently wrote home saying he was being "staked out" and that the guards feared for their own safety.

The violence besetting Iraq continued unabated Monday with the killing of two Sunni Muslim clerics in Baghdad and a car bomb explosion in the northern city of Mosul that killed three people, even as interim Prime Minister Ayad Allawi was making the case for going ahead with national elections scheduled for January.

Practically the only positive news for the Iraqi government was the reported release of 18 members of the Iraqi National Guard who were seized by another insurgent group last week.

On a visit to London, Allawi once again sought to convince the world that Iraq was capable of holding the scheduled election notwithstanding concerns expressed by U.N. Secretary General Kofi Annan.

Allawi, who is due in Washington later this week, told a joint news conference with British Prime Minister Tony Blair Monday that "we have to do it because once democracy is victorious in Iraq, it is a big defeat for terrorists and terrorism. . . .

"Terrorists are coming and pouring in from various countries into Iraq to try and undermine the situation in Iraq," Allawi said. "They're coming from Afghanistan, Pakistan, from Europe, from Morocco, from Syria and so on.

"Iraq is on the front line of fighting these terrorists," he said. "And, God forbid, if Iraq is broken or the will of Iraq is broken, then London would be a target, Washington will be a target, Paris will be a target, Cairo will be a target, as we have seen in the past."

The killing of the Sunni clerics raised again the potential for sectarian warfare between members of Iraq's majority Shiite Muslim community and the country's Sunni Muslims.

The clerics, according to the al-Jazeera satellite television network, were Hazim Zaidi, whose body was found late Sunday in front of a mosque in the Sadr City section of Baghdad, and Muhammed Jadwa, who was shot Monday as he was leaving a mosque in western Baghdad.

Both were members of the Association of Muslim Scholars, an influential Sunni group. No group claimed responsibility for the death of the two clerics.

Other clerics from the association have been killed in the past year by unknown assailants. The motives remain uncertain, although U.S. officials have in the past cited a desire by militants linked to insurgent leader Zarqawi to stir sectarian conflict in Iraq.

The killings and the new kidnappings followed the arrest Sunday of an aide to radical Shiite cleric Moqtada Sadr by U.S. and Iraqi security forces. The aide, Hazim Araji, is a spokesman for Sadr in the Kadhimiya district of Baghdad.

Al-Jazeera reported Sunday that a previously unknown organization calling itself the Brigades of Mohammed bin Abdullah was threatening to kill the Iraqi National Guard members unless the Sadr aide was released. The same network said Monday afternoon that the guardsmen had been released in response to a demand by Sadr, who had denounced the kidnapping through a spokesman.

Also Monday, U.S. warplanes struck the Sunni insurgent stronghold of Fallujah, west of the capital, killing two people and wounding three, news services reported. The U.S military said the missile strike hit Tawhid and Jihad militants who were setting up fortifications in the city.

The Iraqi Health Ministry said that in the 24 hours before Monday's airstrike, seven Iraqis were killed and 14 wounded in U.S. attacks on Fallujah.

Branigin reported from Washington. Staff writer Fred Barbash in Washington and correspondent Rajiv Chandrasekaran in Baghdad contributed to this article.