BAGHDAD, Iraq - Relatives pleaded today for the release of two Americans and a Briton as a deadline loomed for their beheading. In Baghdad, two Sunni Muslim clerics from a powerful conservative group were assassinated within hours of each other.

The Tawhid and Jihad group, led by Jordanian terror mastermind Abu Musab al-Zarqawi, has threatened to behead Americans Jack Hensley and Eugene Armstrong and Briton Kenneth Bigley today unless Iraqi women are released from two U.S.-controlled prisons here.

The threat came in a video released Saturday, which showed the three construction contractors blindfolded and said they would be killed in 48 hours, though no exact time for the deadline was given. The three construction contractors were snatched on Thursday from their Baghdad home.

The British government and Bigley's brother, Philip, appealed for their release in statements broadcast repeatedly today on the Arab satellite television station Al-Arabiya.

"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."

Hensley's wife, Patty, appeared on the Arab television station Al-Jazeera and said her husband, like all Americans in Iraq, was there to help the Iraqi people.

The appeals came a day after another group posted a grisly video on the Internet showing the beheading of three other hostages - said to be Iraqi Kurd militiamen. The bodies of the three were later found by a road outside the northern city of Mosul.

Tawhid and Jihad has claimed responsibility for the slaying of three hostages in the past, including the beheading of American Nicholas Berg, who was abducted in April. The group has also said it is behind a number of bombings and gun attacks.

In the latest kidnapping, it is demanding the release of Iraqi women from Abu Ghraib and Umm Qasr prisons. Abu Ghraib is the prison where U.S. soldiers were photographed sexually humiliating male prisoners. The U.S. military says no women are held at either facility, though it says it is holding two female "security prisoners" elsewhere.

Insurgents waging a 17-month campaign of violence against U.S. and Iraqi forces have used kidnappings and spectacular bombings as their weapons of choice to undermine the interim government of Prime Minister Ayad Allawi and push the United States out of Iraq.

It was not immediately known who was behind the gunning-down of two Sunni clerics Sunday night and today in Baghdad. The two clerics belonged to the Association of Muslim Scholars, a grouping of conservative clerics that opposes the U.S. presence in Iraq and has emerged as a powerful representative of Iraq's Sunni minority.

Gunmen shot and killed Sheik Mohammed Jadoa al-Janabi as he entered a mosque in Baghdad's predominantly Shiite al-Baya neighborhood to perform noon prayers today, the association said.

The previous night, gunmen attacked the car of Sheik Hazem al-Zeidi as he left a mosque in another largely Shiite neighborhood of Baghdad, Sadr City. Al-Zeidi was killed and two of his bodyguards were taken hostage for several hours before being released Monday, said Sheik Abdul-Sattar Abdul-Jabbar, a senior member of the association.

Sadr City has a Shiite Muslim majority, but Sunni Muslims have at least 10 mosques in the neighborhood.

In Baghdad, where Shiites and Sunnis are roughly equal in number, attacks have taken place against places of worship belonging to both communities.

The association is believed to have contacts with Sunni insurgents and has interceded often in the past to win the release of foreign hostages.

More than 100 foreigners have been kidnapped in Iraq, some for lucrative ransoms, and at least 26 of them have been executed. At least five other Westerners are currently being held hostage here, including an Iraqi-American man, two female Italian aid workers and two French reporters.

In the southern Shiite holy city of Najaf, a spokesman of renegade Shiite cleric Muqtada al-Sadr called for the release of 18 Iraqi National Guard members, purportedly kidnapped by masked gunmen who are demanding the release of a detained al-Sadr aide.