An al-Qaida front group warned the United States on Monday to halt its expanding search for three missing American soldiers "if you want their safety." The Pentagon acknowledged for the first time it believes the servicemen are in al-Qaida hands.

The statements came as thousands of U.S. and Iraqi troops swept through farmhouses, fields and palm groves south of Baghdad in hopes of finding the soldiers — last seen before a pre-dawn attack Saturday. Four Americans and one Iraqi soldier were killed in the ambush.

For a third day, jets, helicopters and unmanned surveillance aircraft crisscrossed the skies over the sparsely populated farm area near Mahmoudiya, 20 miles south of Baghdad. U.S. and Iraqi troops — backed by dog teams — searched vehicles and pedestrians. Other teams peered into crawl spaces and probed for possible secret chambers in homes.

Residents complained of random detentions and homes being ransacked as the hunt drew in more troops and brought taunting messages from the presumed captors.

In a Web posting, the Islamic State of Iraq, demanded that the Americans stop the search because it will "lead to nothing but exhaustion." "Your soldiers are in our hands. If you want their safety, do not search for them," the statement said.

It also suggested that the weekend ambush was in revenge for the rape-murder of 14-year-old Abeer Qassim al-Janabi by American soldiers in the area last year. Five soldiers have been charged in the case, and three have pleaded guilty.

In a statement Monday, chief U.S. military spokesman Maj. Gen. William Caldwell said the U.S. was using "every asset and resource available" to find the missing soldiers.

Last June, al-Qaida claimed responsibility for the deaths of two U.S. soldiers whose mutilated bodies were later found in the same area where the weekend ambush occurred. Later, al-Qaida statements also linked those killings to the rape-murder of the 14-year-old Sunni girl.

In another Web statement, a rival coalition of Sunni resistance groups — the Jihad and Reform Front — accused al-Qaida of killing 12 of its senior members in Baghdad's Dora neighborhood about 15 miles north of the search area.

The coalition claimed to represent the Islamic Army of Iraq, the 1920 Revolution Brigade and the Mujahedeen Army and apparently signaled new rifts within insurgent ranks in Iraq.

During the search Monday, U.S. and Iraqi forces exchanged fire with gunmen near the town of Youssifiyah, killing two and injuring four, an Iraqi army officer said.

The officer, who spoke on condition of anonymity because he was not supposed to release information, said about 100 suspects had been detained. The U.S. military would not comment.

In a statement Monday, the U.S. command said the attack on Saturday occurred at 4:44 a.m., but a rescue force took an hour to reach the site because the area was rigged with bombs. The military said the quick reaction force found three roadside bombs hidden near the site and feared insurgents were preparing an ambush.

"You can't just rush into an area ... or your first responders could become casualties as well," said spokesman Lt. Col. Christopher Garver. "You don't know if somebody is waiting there with a rocket-propelled grenade and if they shoot a helicopter down then it becomes a much bigger problem."

Garver said about 4,000 U.S. troops were participating in the search operation.

Elsewhere, five other U.S. troops were killed in separate attacks Monday in Baghdad and surrounding areas south of the capital. A sixth soldier died Monday of non-combat related causes, the U.S. military said.

That raised number of American service members to have died this month at least to 47.

In southern Iraq, a roadside bomb in Basra killed one Danish soldier and wounded five other Danes and their Iraqi translator, the Danish military said.

Seven Danish soldiers have been killed in Iraq since the war began. In February, the Danish government said it would withdraw its 460-member contingent from Basra by August and replace it with a smaller helicopter unit.

In other violence, at least 52 Iraqis were killed or found dead after a series of bombings, shootings and mortar attacks, all but 11 in Baghdad.

PHOTO CAPTION

An Iraqi soldier holds a machine gun as he mans a checkpoint in Baghdad May 13, 2007. (Reuters)

Reuters (summarized)