MONROVIA, Liberia - American Marines shot and killed three Liberians firing toward the U.S. Embassy yesterday as fierce battles engulfed the capital, littering its streets with bodies a day after a cease-fire collapsed.

The shootings, the first exchange of gunfire involving U.S. troops since Liberia's civil war reignited in early April, came as new factional fighting left the 10-day-old, U.S.-brokered truce in tatters.

One U.S. Marine was grazed - possibly by a cartridge from his own weapon - but required no medical attention, said two senior defense officials in Washington, speaking on condition of anonymity.

At an army barracks a few blocks from the embassy, the floor of a clinic was awash in blood as scores of victims were carried in or dragged themselves through the door. Doctors said at least three people had died and 60 were wounded, many civilians.

Fire from mortars, machine guns and rocket-propelled grenades echoed through the city, forcing George Moose, the U.S. assistant secretary of state for African affairs, to cancel talks at the embassy with government, peacekeepers and factional leaders.

Moose, in a telephone interview from the embassy, warned of a ''major catastrophe'' if the fighting continued. He blamed warlord-turned-politician Charles Taylor, whose standoff with a rival April 6 provoked two weeks of warfare culminating in the cease-fire.

''Taylor will be held responsible,'' Moose said. ''If he persists in taking power, he should expect a strong international reaction.''

In Monrovia's Mamba Point diplomatic section, where the embassy is located, rival factions battled for control through the afternoon. At least nine bodies were scattered on the streets of the beachfront neighborhood. It was unclear whether those killed died while fighting or were caught in crossfire.

Similar chaos spread across the capital. Chawki Bsaibes, a Lebanese businessman who runs the Mamba Point Hotel, said yesterday's fighting was the worst he had seen in the city.

In separate incidents at the embassy, three Liberians were killed and one wounded by U.S. Marine gunfire, defense officials in Washington said. The most serious exchange took place at a guard post near U.S. Ambassador William Milam's residence, Pentagon spokesman Kevin Bacon said.

In all, the Marines counted four separate shooting incidents beginning in late afternoon. In two cases, the Marines could see their attackers and returned fire using M-16 automatic rifles and M-60 machine guns.

''We don't know what (the attackers) were aiming at. Fire came in and it was returned,'' Bacon said. The attackers' identities were not known.

The Marines are part of a group of 270 - from an offshore force of almost 3,000 - who are protecting the embassy, where staff has been reduced to 18 people.

At the clinic, located at the Barclay Training Center army barracks, rocket-propelled grenades smashed into the grounds as doctors treated patients. Civilians wailed in pain and thrashed on floors and flimsy stretchers.

One foreigner, French freelance photographer Axel Grousset, was treated for shrapnel wounds suffered when a grenade fell into the street near his vehicle. His injuries were not believed to be serious. By about 5 p.m., fighting in the capital had subsided, and by nightfall the area was quiet.

The latest round of fighting broke out Monday at Barclay, where warlord Roosevelt Johnson has been holed up since April 6 with hundreds of supporters trying to evade arrest by government troops.