NEAR FALLUJAH, Iraq - Insurgents set off at least two car bombs and attacked a police station Saturday in the central Iraqi town of Samarra, killing at least 20 people and wounding 23 in what could be an effort to take pressure off Fallujah.

The violence in Samarra, a city that has been billed as a success story in the effort to restore order in Iraq, came as U.S. warplanes intensified attacks on Fallujah in preparation for a wider assault.

For the past three nights, long convoys of American soldiers have rolled onto a dust-blown base on the outskirts of the guerrilla stronghold, a symbol of Iraqi resistance.

More than 10,000 American soldiers and Marines are massed for an expected offensive, and Iraq's prime minister warned the "window is closing" to avert an attack.

U.S. planes dropped five 500-pound bombs at several targets in the city early Saturday, including a factory as well as suspected weapons caches. The drone of U.S. aircraft heading toward Fallujah could be heard over Baghdad. The U.S. military said the main highway into Fallujah has now been completely sealed off.

In Samarra, 60 miles northeast of Fallujah, armed militants stormed a police station, leaving 12 policemen dead and injuring one. In other attacks, a suicide car bomber detonated explosives inside a stolen police car near the mayor's office, a second car bomb exploded near a U.S. base and a mortar fell on a crowded market.

The dead included an Iraqi National Guard commander, Abdel Razeq Shaker al-Garmali, hospital officials said. The town's mayor was reportedly injured in the car bombing.

Residents said U.S. forces, using loudspeakers to make the announcement, imposed an indefinite curfew on Samarra. American warplanes and helicopters were heard roaming overhead.

U.S. and Iraqi forces had seized control of the city in September, and Iraqi officials have pointed to it as an example of how the Americans and the Baghdad government can restore calm in restive areas.

The new violence could be a rebel move to take pressure off Fallujah or exploit the situation with so much attention directed on the city.

U.S. commanders here have been coordinating plans either to fight their way into Fallujah or isolate it from the rest of Iraq's Sunni Muslim heartland.

If they fight, American troops will face an estimated 3,000 insurgents dug in behind defenses and booby traps. Military planners believe there are about 1,200 hardcore insurgents in Fallujah - at least half of them Iraqis. They are bolstered by insurgent cells with up to 2,000 fighters in the surrounding towns and countryside.

In Brussels, Belgium, Iraq's interim prime minister, Ayad Allawi, warned that the "window really is closing for a peaceful settlement" in Fallujah. Allawi must give the final go-ahead for the offensive, part of a campaign to curb the insurgency ahead of national elections planned for January.

Sunni clerics have threatened to boycott the election if Fallujah is attacked, and U.N. Secretary-General Kofi Annan has warned U.S., British and Iraqi authorities that a military campaign and "increased insurgent violence" could put elections at risk.

Iraqi authorities closed a border crossing point with Syria, and U.S. troops set up checkpoints along major routes into the city. Marines fired on a civilian vehicle that did not stop, killing an Iraqi woman and wounding her husband, according to the U.S. military and witnesses. The car didn't notice the checkpoint, witnesses said.

The insurgents struck back, killing one U.S. soldier and wounding five in a rocket attack. Clashes were reported at other checkpoints around the city and in the east and north of the city late in the day. An AC-130 gunship fired at several targets as U.S. forces skirmished with insurgents, the U.S. army said.

The violence came a day after two Marines were killed and four were wounded in fighting west of Baghdad.