NEAR FALLUJAH, Iraq - Backed by a barrage from warplanes and artillery, U.S. troops fought their way into the western outskirts of Fallujah today, seizing a hospital and two bridges over the Euphrates River in the first stage of a major assault on the insurgent stronghold.

The U.S. military said Iraqi troops captured 38 people, including four foreigners when they swept into the first objective: Fallujah's main hospital, which the military and Iraqi Prime Minister Ayad Allawi said was under insurgent control.

Iraqi soldiers stormed through the facility, blasting open doors and pulling handcuffed patients into the halls in search of gunmen.

Allawi said he had given the green light for international and Iraqi forces to launch the long-awaited offensive against Fallujah, considered the strongest bastion of Iraq's Sunni insurgents. "We are determined to clean Fallujah of terrorists," he said.

Allawi initially said 38 people were killed in the hospital seizure, but the U.S. military later said the people were captured.

Throughout the morning, artillery and mortars pounded targets in Fallujah and on its outskirts, and a U.S. jet swooped low to fire rockets at insurgent positions. An AC-130 gunship raked the city all night long with cannon fire, and before dawn, four 500-pound bombs were dropped, raising orange fireballs over the city's rooftops.

Commanders said the toughest fight was yet to come: when American forces cross to the east bank of the Euphrates and enter the main part of Fallujah - including the Jolan neighborhood where insurgent defenses are believed the strongest.

In the first foray across the river into Fallujah proper, Marines secured an apartment building in the northwestern corner of the city by noon, said Capt. Brian Heatherman, of the 3rd Battalion 1st Marine Regiment.

"The Marines have now gained a foothold in the city," said Heatherman, 32, from Laguna Niguel, Calif.

He said there were some Iraqi casualties as the troops seized the building, where Marines found an improvised bomb hanging above a doorway - one of the many variety of booby traps they expect to come across in the urban battle.

Marine commanders have warned the offensive against Fallujah could bring the heaviest urban fighting since the Vietnam war. Some 10,000 U.S. Marines, Army soldiers and Iraqi forces are around Fallujah, where commanders estimate around 3,000 insurgents are dug in. More than half the civilian population of some 300,000 people is believed to have fled already.

U.S. and Iraqi commanders have vowed to stamp out Sunni Muslim guerrillas controlling Fallujah and other cities north and west of Baghdad ahead of vital January elections.

Allawi's government announced on Sunday that it was imposing a 60-day state of emergency across Iraq - except for the Kurdish-run north.