Fallujah, Iraq ? U.S. Marine infantrymen fought with insurgents in Fallujah on Thursday as warplanes and tanks bombarded guerrilla positions in the heaviest fighting here in weeks. The clashes raged as nearly 1,000 residents returned to the devastated city for the first time since U.S. troops drove out most of the militants last month.

Hours later in Mosul, Defense Secretary Donald H. Rumsfeld paid a surprise visit to soldiers wounded in a deadly attack at a U.S. air base.

At least three Marines were killed in the fighting in Fallujah, underlining how far the city and surrounding area are from being tamed as the United States and its Iraqi allies try to bring quiet before national elections Jan. 30.

In the center of Fallujah, F-18s dropped several bombs, sending up plumes of smoke. Tank and machine gun fire could be heard to the south, while howitzers at Camp Fallujah southeast of the city boomed throughout the day. The guns fired illumination rounds after dark to help Marines on the ground spot attackers.

The military would not give specific figures for casualties in Fallujah, saying only that three Marines were killed in action Thursday in Anbar province, which surrounds the city. But a U.S. official, speaking on condition of anonymity, confirmed the three deaths occurred in the Fallujah clashes.

American troops have also faced growing violence in Mosul, the northern city where a blast ripped through a mess tent at a military base Tuesday, killing 22 people — most of them Americans. The suicide bomber believed to have carried out the attack was probably wearing an Iraqi military uniform, a U.S. general said Thursday.

Arriving in darkness early today, Rumsfeld walked immediately from his plane to a combat surgical hospital where many of the bombing victims were treated.

In an interview aboard the C-17 cargo plane that brought him to Mosul, Rumsfeld said he’d been planning to visit U.S. troops here long before the deadly attack.

“The focus of the trip is to thank the troops and wish them a Merry Christmas,” he said.

The FBI has joined the investigation into the blast as the military reassesses security at bases across Iraq in light of the bomber’s success in apparently slipping into the camp and entering a tent crowded with soldiers eating lunch.

While the fighting raged, hundreds of Fallujans spent hours lined up at a checkpoint northwest of the city, one of four entry points where people had to prove they lived in the Andalus district, a mostly commercial area in the heart of Fallujah that was the first neighborhood opened to residents.

Most of the people showed up on foot or shuttle buses, not having gotten word that authorities had changed their minds about allowing cars into the city. U.S. officials had wanted to keep vehicles out, to lessen the chances for car bombings, but Iraq’s interim government insisted people be permitted to bring in their cars.

Officials said few people stopped by humanitarian centers set up around the city to offer food and other supplies. Huge tanks holding drinking water have been erected across Fallujah because the water system is still out of commission.