U.S. President George W. Bush on Tuesday condemned the beheading of an American hostage in Iraq.

A website used by Islamic militant groups posted a video on Monday showing the beheading of Eugene Armstrong.

Meeting with Iraqi Prime Minister Ayad Allawi on Tuesday at the United Nations building in New York, Bush said, "We will not allow these thugs and terrorists to decide your fate and decide my fate."

The CIA says that the voice on the videotape was Islamic militant Abu Musab al-Zarqawi and that al-Zarqawi was likely the person who killed Armstrong.

Armstrong was abducted on Thursday along with two other construction contractors, American Jack Hensley and Briton Kenneth Bigley.

The body of Armstrong was found on Monday a few blocks from where he lived in Baghdad.

The group demanded that all Muslim women in U.S. custody in Iraq be released. The U.S. military says it's holding only two women: 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.

British Prime Minister Tony Blair condemned the killing on Monday, and told a news conference that "our response has not got to be to weaken ... Our response has got to be to stand firm."