Near Fallujah, Iraq ? U.S. warplanes pounded Fallujah late Friday in what residents called the strongest attacks in months, as more than 10,000 American soldiers and Marines massed for an expected assault. Iraq’s prime minister warned the “window is closing” to avert an offensive.

Residents reached by telephone said the aircraft were striking targets in the central city market that had not been hit since April as well as neighborhoods in the north, south and east of Fallujah. There was no confirmation from U.S. officials.

Earlier Friday, residents said U.S. planes dropped leaflets urging women and children to leave the city, 40 miles west of the capital, Baghdad.

As pressure mounted on the guerrilla stronghold, 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 Fallujah late in the day.

In Baghdad, a huge column of black smoke rose over the city’s Karrada district after midnight, and residents said they heard heavy gunfire, presumably between police and militants. No one answered the phone at the local police station.

For the past three nights, long convoys of American soldiers from Baghdad and Baqouba have rolled onto a dust-blown base on the outskirts of Fallujah, a city that has become the symbol of Iraqi resistance. U.S. commanders here have been coordinating plans either to fight their way into the city 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 is closing” to avert an assault on 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.

A U.S. Marine from the 1st Division checks ammunition Friday at a base outside Fallujah, Iraq. More than 10,000 U.S. troops have taken positions around the rebel-controlled city of Fallujah, bolstering the U.S. Marine units expected to lead a joint Army-Marine assault there.

U.S. aircraft struck targets around Fallujah in numerous raids starting late Thursday and continuing into Friday night. Targets included a system of barriers rigged with bombs, a command post, suspected fighting positions and a weapons cache, according to Lt. Nathan Braden of the 1st Marine Division.

Late Friday, U.S. jets went into action again, striking several targets in the Jolan sector of northern Fallujah, residents said. Jolan is considered one of the most heavily defended neighborhoods in the city.

As the night dragged on, the attack was expanded to targets in many areas of city, according to residents. Artillery fire could also be heard. The drone of U.S. aircraft heading toward Fallujah could be heard over Baghdad.