BAGHDAD, Iraq - U.S. troops fought a gunbattle with insurgents along a busy street in Baghdad today, sending passers-by scurrying for cover, witnesses said, while five U.S. troops were reported killed in separate clashes in a volatile western province as insurgents step up attacks ahead of next month's elections.

The violence came a day after gunmen ambushed a bus carrying unarmed Iraqis to work at a U.S. ammo dump near Tikrit, killing 17 and raising the death toll from three days of intensified insurgent attacks to at least 70 Iraqis.

The attacks, focused in Baghdad and several cities to the north, appeared to be aimed at scaring off those who cooperate with the American military - whether police, national guardsmen, or ordinary people just looking for a paycheck.

They also have targeted Kurdish militiamen and Shiite worshippers in a possible bid to foment sectarian and ethnic unrest.

The latest fighting in Baghdad broke out after armed rebels appeared on the busy Haifa Street, saying they were hunting for Iraqis collaborating with U.S.-led forces.

Witnesses said they shot and killed a man they claimed was working for the Americans. Rebels also were seen on a square just three blocks from the heavily fortified Green Zone that houses Iraq's interim government and the U.S. Embassy.

The U.S. military had no immediate comment, but witnesses said U.S. troops supported by armored vehicles attacked the gunmen.

Haifa Street, a thoroughfare running through central Baghdad, has been the scene of frequent clashes between U.S. troops and resistance fighters.

Earlier today, three insurgents were killed and four wounded in clashes with U.S. forces in Haditha in the volatile Anbar province, according to Dr. Bassem Izaldeen, of Haditha Hospital.

The 1st Marine Expeditionary Force also released a statement saying three soldiers attached to the Marines died in two incidents Sunday in the western province, which includes the battleground cities of Fallujah and Ramadi. Earlier, the military said two Marines had been killed in action in Anbar on Friday.

The killings raised the number of U.S. troops killed since Friday to 11 and brought to at least 1,276 the number of U.S. troops to have died since the war began in March 2003.

The attacks came just weeks after the United States launched major offensives aimed at suppressing guerrillas ahead of crucial elections set for Jan. 30. But the insurgents have struck hard in recent days, showing they are just as capable as ever despite the American-led campaign.

Sunday's bloodshed began when gunmen opened fire at the bus as it dropped off Iraqis employed by coalition forces at a weapons dump in Tikrit, said Capt. Bill Coppernoll, spokesman for the Tikrit-based U.S. 1st Infantry Division. Coppernoll said 17 people died and 13 were wounded in the attack.

Survivors said about seven guerrillas were involved, emptying their clips into the bus before fleeing.