"We have conducted more than 450 tactical interviews and detained 11 individuals" as of Monday night, U.S. military spokesman Lt. Col. Christopher Garver said without elaborating.
"We are working with the Iraqi army and the Iraqi police in searching for our missing soldiers. We're also communicating with the local population for information, for support and the local population continues to be helpful in providing tips," Garver said.
For a fourth day, jets, helicopters and unmanned surveillance aircraft crisscrossed the skies over the sparsely populated farm area near Mahmoudiya, 20 miles (32 kilometers) 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.
On Monday, the Islamic State of Iraq - an al-Qaida front group that has claimed to have captured the soldiers - warned the U.S. to halt its search by about 4,000 troops, and the Pentagon acknowledged for the first time that it believes the soldiers are in terrorist hands.
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 a weekend ambush had occurred.
If all three soldiers now missing are taken hostage alive, it would be the biggest single abduction of U.S. soldiers in Iraq since March 23, 2003, when Pvt. Jessica Lynch and six others were captured in an ambush near Nasiriyah in which 11 Americans were killed.
They were last seen before a pre-dawn ambush Saturday that destroyed several Humvees in a U.S. convoy and killed four Americans and one Iraqi soldier traveling with them.
At 9:15 a.m. Tuesday a bomb hidden in a minibus leaving a bus stop on a main road in Mahmoudiya exploded, wounding three Iraqi passengers, police said.
Al-Qaida has been active for years in the string of towns and villages in the area south of the capital. The mostly Sunni region is known as the "triangle of death" because of frequent attacks on U.S. and Iraqi forces as well as Shiite civilians traveling to shrine cities in the south.
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.
On Tuesday, an Iraqi interpreter working with the U.S. soldiers said the coalition's search was focusing on rural areas outside Mahmoudiya and that life was proceeding as normal in the city.
But he also said Iraqi civilians being stopped for questioning by U.S. forces appeared nervous that they could be attacked by insurgents later, if they were seen cooperating with the coalition. The interpreter spoke on condition of anonymity out of concern for his own security.
The area around Mahmoudiya has long been especially volatile because Saddam Hussein recruited members of Sunni tribes there into his elite Republican Guard and intelligence services. Many of them were believed to have joined the insurgency after Saddam's regime collapsed in the 2003 U.S.-led invasion. U.S. officers also say extremists have fled Baghdad for surrounding areas to escape the three-month Baghdad security crackdown.
In other operations or violence in Iraq on Tuesday,
Unidentified gunmen killed Iraqi army Col. Raed Ahmed Shihab in Baghdad as he drove in the city, police said. He had worked for the Iraqi ministry of defense.
The U.S.-led coalition detained 10 suspected insurgents during raids targeting al-Qaida in Iraq and another hard-line militant Sunni group, the Ansar al-Sunna Army, in the cities of Mosul, Fallujah and an area near the U.S. Air Force base of Taji, north of the capital, the military said.
A roadside bomb apparently hit a U.S. convoy in the Kamalia area of southeastern Baghdad. Associated Press TV video showed one of the convoy's trucks burning and two U.S. Black Hawk helicopters circling overhead. Later, when the fire went out, Iraq men and young boys were shown on the footage looting what remained of the truck.