FALLUJAH, Iraq -- U.S. warplanes pounded Fallujah with 500-pound laser-guided bombs Wednesday and Marines battled insurgents near a train station and in neighborhoods that had seemed to be quieting. American forces decided to delay potentially dangerous patrols into the besieged city.

The violence, carried on live television with images of fiery destruction, came as the United States was under increasing international pressure to prevent a revival of the bloodshed seen in the city west of Baghdad during the first two weeks of April.

"Violent military action by an occupying power against inhabitants of an occupied country will only make matters worse," Secretary-General Kofi Annan said. "It's definitely time, time now for those who prefer restraint and dialogue to make their voices heard."

Commanders in Iraq said the Marines were responding to guerrilla attacks and that the military was sticking to a more than two-week-old halt in offensive operations to allow negotiations.

"Even though it may not look like it, there is still a determined aspiration on the part of the coalition to maintain a cease-fire and solve the situation in Fallujah by peaceful means," Brig. Gen. Mark Kimmitt said in Baghdad.

Guerrilla attacks broke out in at least three neighborhoods of Fallujah that had been relatively quiet during the past three days. And the U.S. response intensified: when a Marine was wounded, warplanes dropped 10 laser-guided bombs -- most of them 500-pound bombs but at least one 1,000 pound -- on buildings that were the source of guerrilla fire, Lt. Col. Brennan Byrne said.

At least twice, AC-130 gunships opened up on guerrilla positions with their heavy cannons.

Sound of battle

Throughout the day, the sound of each battle was heard -- the rattle of gunfire and the thud of mortars -- then came the noise that often marked Marine strikes to put an end to the fight: heavy explosions, raising flames and palls of smoke.

Guerrillas fired on a train station just outside the city's northern edge, prompting a battle in the Golan neighborhood, an insurgent bastion. Fighting also erupted on the northeast, southeast and in the center of the city.

The extent of the battle was difficult to gauge. Witnesses reported at least 25 buildings wrecked by fighting. Hospitals only counted 10 wounded Iraqis, but ambulances could not reach areas where fighting was going on, and residents reported large numbers of dead and wounded.

At the White House, President Bush said "most of Fallujah is returning to normal."

But the fighting was causing international concern. Escalating violence in the Middle East prompted already nervous investors to sell stocks sharply lower Wednesday, sending the Dow Jones industrials down more than 110 points.

Late in the day, Byrne announced that Marine patrols into the city due to start today had been delayed a day.

The United States decided over the weekend to send in the patrols of Marines and Iraqi security forces to establish a semblance of control over the city.

But with tensions rising, Marines moving on foot through the city streets would almost inevitably draw guerrilla attacks -- which could then trigger heavier fighting.

Several families were seen fleeing the city Wednesday during the battles, the latest to puncture a tattered cease-fire in Fallujah.

"I was pinning some hope on the truce. The American air bombing dashed my hopes," Ali Muzel said as he escorted his wife and five children to Baghdad.

A third of the city's 200,000 residents fled earlier.

Across Iraq, attacks are down, compared with the first two weeks of April, as U.S. officials try negotiate solutions for Fallujah and with militiamen loyal to radical Shiite cleric Muqtada al-Sadr in the south. But violence still flares regularly.

A U.S. soldier was killed in an ambush Tuesday near the northern city of Tel Afar, the military reported.

The soldier's death brought to 116 the number of Americans killed in combat this month, the bloodiest month for U.S. forces in Iraq. At least 725 U.S. troops have died in Iraq since the war began in March 2003. Up to 1,200 Iraqis also have been killed this month.

Also Wednesday, a senior U.S. official said investigators have recommended administrative punishment for a number of commanders after allegations that prisoners were abused at Baghdad's Abu Ghraib prison. The official, speaking on condition of anonymity, would not give details on the recommended punishments or say how many commanders faced action.

Six soldiers at the prison face criminal charges, Kimmitt said.

Saddam Hussein's 67th birthday was Wednesday, his first in U.S. detention since being captured in December. In his hometown of Tikrit, there were no apparent signs of celebration, and schools and universities were closed.

In southern Iraq, gunmen ambushed a Ukrainian convoy outside the city of Kut on Wednesday. Two coalition soldiers were killed, Kimmitt said.

Shiite militiamen loyal to al-Sadr drove Ukrainian peacekeepers out of Kut earlier this month, but U.S. troops then swept into the city, pushing out most militiamen.

U.S. troops seek to capture al-Sadr and suppress his militia. On Wednesday, they began gradually expanding operations outside their base in the holy city of Najaf. Soldiers set up checkpoints on the road outside the base -- the main route between the center of Najaf and neighboring Kufa.

Another checkpoint was established on a bridge outside Kufa, near the site of a battle Monday in which the military reported 64 Iraqis were killed. Najaf hospitals reported only 37 people killed.

The military has promised to stay away from sacred Shiite sites at the heart of Najaf. The U.S. base is about three miles from the shrines.