Baghdad, Iraq ? The U.S. military said Monday that three soldiers missing since their patrol was ambushed in the early hours of Saturday have likely been captured by an al-Qaida-affiliated insurgent group.

On the third day of the massive manhunt for the soldiers, the military cited what it called “highly credible intelligence information” in its assessment that the soldiers were in al-Qaida hands, but refused to offer further details.

“We believe they were abducted by terrorists belonging to al-Qaida or an affiliated group,” a military statement said, adding that it did not want to release further details for fear of jeopardizing the search effort.

Hours later, the Islamic State of Iraq, the al-Qaida umbrella organization that had already claimed it is holding the men, warned the U.S. on its Web site to call off the search if it wants the men to remain safe.

“We say to you, the search operations you are conducting will not result in anything other than exhaustion and misery for you,” the statement said. “Your soldiers are in our hands, so if you are concerned for their safety, stop searching for them.”

The statement boasted that the capture of the U.S. soldiers had “humiliated” the U.S. military, but the warning to stop the search suggested that whoever is holding the soldiers is feeling the heat from the intensive search under way near the area where they were seized.

More than 4,000 U.S. and Iraqi soldiers backed by helicopters, aircraft and unmanned spy planes have been scouring villages, farms and the desert landscape for the soldiers, who went missing when their patrol was attacked Saturday at a remote location 12 miles west of Mahmoudiyah.

Four American soldiers and one Iraqi soldier were found dead in and near a burning Humvee, but the occupants of a second Humvee, which was also ablaze, were nowhere to be seen.

Military spokesman Maj. Gen. Caldwell pledged that the military would not give up the hunt and said he wanted to reassure the families that everything possible is being done to find the soldiers. Helicopters, aircraft, unmanned spy planes and dogs are being used in the hunt involving thousands of U.S. and Iraqi troops, he said.

“Everyone who wears this uniform in combat understands and lives by The Soldiers Creed, and one of its key tenets is ‘I will NEVER leave a fallen comrade,”‘ his statement said. “We live by that creed.”

As the search continued, the U.S. military announced the deaths of five other servicemen in bombing and shooting attacks and a sixth death from noncombat causes.

Four of the deaths were in Baghdad, including two soldiers struck down by gunfire while on a foot patrol, one who was killed in a roadside bombing and an airman who died in a separate bombing attack. A Marine died in combat operations in volatile Anbar province, the military said.