(U-WIRE) WASHINGTON – U.S. military officials declared victory in the former insurgent stronghold of Fallujah Monday, but some resistance remains and violence has spilled to neighboring Sunni Triangle cities.

U.S. military sources reported 38 U.S. casualties and estimates guerrilla deaths at over 1,000 since the offensive began early last week, according to Reuters.

“The city has been seized,” Lt. Gen. John F. Sattler, commander of the 1st Marine Expeditionary Force, said in a statement Monday. “We have liberated the city of Fallujah.”

U.S. and Iraqi soldiers encountered roadside bombs, rockets and gunfire early last week as they fought to the heart of Fallujah in the largest operation since the U.S.-led invasion of Iraq in March 2003. Through the weekend they pounded the city with bombs and artillery in its southern neighborhoods, where the U.S. military said remaining resistance fighters are holding out.

“What you’re seeing now are some of the hardliners,” Maj. Gen. Richard Natonski told the BBC. “They seem to be better equipped than some of the earlier ones. We’ve seen flak jackets on some of them.”

But many insurgents, including rebel leaders, fled the city before the firing of shots in the offensive, according the U.S. military. Resistance has since emerged in other Sunni cities.

The most severe fighting is in Mosul, a city of 1.8 million where several police stations have been laid waste since late last week and news service reports described street fighting between insurgents and U.S. and Iraqi forces.

The U.S. military on Monday said the situation in Mosul is “stable.”

“I expect the next few days will bring some hard fighting,” Brig. Gen. Carter Ham, the senior American commander in the area, told the Washington Post. “The situation in Mosul is tense but not desperate.”

U.S. forces called in air strikes on insurgent positions in Baqubah Monday in response to attacks on police stations in the provincial capital just northeast of Baghdad, according to the Washington Post and wire services.

Insurgents controlled Fallujah until last week after a siege by U.S. Marines was called off in April. Located in the Sunni Triangle area west of Baghdad, the city of 300,000 was described by U.S. and Iraqi officials as a haven for resistance forces determined to destabilize Iraq’s interim government and drive foreign military forces from the country.

U.S. and Iraqi forces are scrambling to provide Fallujah with humanitarian aid. A four-vehicle convoy of the Iraqi Red Crescent carrying humanitarian assistance arrived in Fallujah after the Iraqi and American troops allowed them to pass with some delay.

U.S. forces have set up camps for displaced civilians four miles north of the city, but the military has no apparent plan for transporting civilians from their war-torn homes to the camp, according to Palm Beach Post-Cox News Service.

But U.S. forces have been using loudspeakers to promise safety to civilians who come out of their homes. Marines gave civilians food, water and limited medical care on the spot, but told families they would have to walk to the camp on their own.

NBC News reported Monday that the U.S. military is investigating the killing of a wounded and apparently unarmed Iraqi prisoner inside a mosque during combat operations.

NBC correspondent Kevin Sites witnessed the incident Saturday while assigned to represent a pool of news organizations. Sites said the man was shot by a Marine who appeared to be unaware that the Iraqi was a wounded prisoner and did not pose a threat.

“We’re confident it will be a thorough investigation,” Defense Department spokesman Bryan Whitman told NBC.

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