NEAR FALLUJAH, Iraq (AP) - More than 10,000 U.S. soldiers and Marines have taken positions around Fallujah for an expected assault, as U.S. jets pummeled insurgent targets Friday and troops blocked key roads. Iraq's prime minister warned the "window is closing" to avert an offensive.

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.

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.