BAGHDAD, Iraq - Family members of an American and a Briton held hostage in Iraq appealed for their lives after militants vowed to kill one of them today, a day after beheading a hostage identified as U.S. civil engineer Eugene Armstrong.

The beheading was shown in a nine-minute video posted on the Internet on Monday. In the footage, a sobbing, blindfolded man identified as Armstrong knelt in front of five militants dressed in black. The banner of Abu Musab al-Zarqawi's Tawhid and Jihad, a group linked to al-Qaida, hung on the wall behind them.

The man in the center read a statement, then pulled a knife, grabbed the hostage seated at his feet and sliced his head off. The victim screamed and blood poured from his neck. Al-Zarqawi, a Jordanian who is the most wanted militant in Iraq, purportedly conducted the beheading himself.

The speaker then threatened to kill at least one more hostage in 24 hours unless all Muslim women are released from U.S. custody in Iraq. The group still claims to be holding American Jack Hensley and Briton Kenneth Bigley, construction contractors abducted along with Armstrong from their Baghdad home last week.

The U.S. military says the only two women in its custody in Iraq are two female security prisoners: Dr. Rihab Rashid Taha, a scientist who became known as "Dr. Germ" for helping Iraq make weapons out of anthrax, and Huda Salih Mahdi Ammash, a biotech researcher known as "Mrs. Anthrax."

The speaker, whose voice resembled al-Zarqawi's, said Tawhid and Jihad was taking revenge for women Iraqi prisoners and called President Bush "a dog."

"You, sister, rejoice. God's soldiers are coming to get you out of your chains and restore your purity by returning you to your mother and father," he said.

In Georgia, Patty Hensley, the wife of Jack Hensely, pleaded with his captors to open lines of communication and spare his life.

"I understand their political agenda, but what I need them to understand is the man who I have been with for 23 years, who is the father of our 13-year-old daughter, who does not understand this situation, why someone would want to hurt her father," Patty Hensley said in an interview with CNN. "I would plead with them to please realize this man does not deserve this fate."

In London, the Bigley family appealed to British Prime Minister Tony Blair to meet the captors' demands.

"I ask Tony Blair personally to consider the amount of bloodshed already suffered," Craig Bigley, 33, said in a videotaped statement. "Please meet the demands and release my father - two women for two men. ... Only you can save him now. You have children and you will understand how I feel at this time."

Philip Bigley pleaded with his brother's captors to free his brother and Hensley.

"We are begging you not to kill them," he said. "We are begging you to find a solution, a compromise, that will help to save two lives, innocent lives."

Blair condemned the kidnappings at a news conference Monday.

"Our response has got to be to stand firm, to say, whatever the differences over the Iraq conflict, there is a clear right and wrong on these issues, and that is to be with the democrats and against the terrorists," said Blair.

Armstrong's body was found Monday a few blocks from where he lived in the leafy west Baghdad neighborhood of Mansour, officials and witnesses said today. Iraqi police found the corpse near a highway overpass and informed U.S. troops, the officials said on condition of anonymity. U.S. troops sealed off the area.

In Washington, a U.S. official, speaking on condition of anonymity, said Armstrong's body had been recovered, but the official would provide no details.

Also today, a car bomb exploded on Baghdad's airport road as a U.S. military convoy was passing by, wounding four American soldiers and damaging a Bradley fighting vehicle and a Humvee, the U.S. military and witnesses said.

Several civilian vehicles also were wrecked in the blast on the main highway leading to the capital's international airport, said Maj. Phil Smith of the 1st Cavalry Division.

Police said there were also civilian casualties in the blast but could not give any exact figures.