FALLUJAH, Iraq – American Marine infantrymen fought with insurgents in Fallujah yesterday 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 American troops drove out most of the rebels last month.

Hours later in Mosul, Defense Secretary Rumsfeld paid a surprise visit to soldiers wounded in a deadly attack at an American 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 America and its Iraqi allies try to bring quiet before national elections January 30.

In the center of Fallujah, F-18s dropped several bombs, sending up plumes of smoke. Tank and machinegun 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 yesterday in Anbar province, which surrounds the city. But an American 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, an American general said yesterday.

Arriving in darkness early today, Mr. 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, Mr. Rumsfeld said he’d been planning to visit American 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.

Iraqi National Guardsmen hung small, colorful posters on concrete barriers that showed a crying girl and the words in Arabic, “Zarqawi killed her parents,” a reference to Jordanian terrorist Abu Musab al-Zarqawi, who was believed to have been in Fallujah before the American attack. Another poster showed a father lifting his daughter to put a ballot in a voting box, meant to encourage people to vote January 30.

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. American 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.

By the end of the day, 921 people had passed through the four checkpoints, 16 were turned back and one was detained, an American military spokeswoman, Major Naomi Hawkins, said.

Officials said people may not have known they were allowed to go home, and more might come after announcements during weekly prayers at mosques today. They also said some people were waiting to hear whether it was safe to return – a question made more acute with the fighting.

“The word is slow to get out,” said Lieutenant Colonel Michael Paulk, a military spokesman. “They want to hear reaction from people who are coming to the city.”

Many may be in for a shock. Marines have said many people staying in refugee camps near Fallujah did not seem to be aware of the extent of damage. Few buildings were left unscarred in the American offensive, which began November 8.

“This is all that’s left of my property,” one returnee said yesterday, waving a dusty blanket.

In footage by Associated Press Television News, the corpse of an elderly woman was visible in a destroyed house, lying face down in her black robe. It was not clear how long ago she was killed.

Others seemed undeterred by the destruction.

“We want to go back to Fallujah. We want to see whether our houses were looted or not,” a man said. “Even if our houses were destroyed, we are ready to set up tents inside Fallujah.”