Insurgents launched attacks in Iraq's third-largest city Thursday, raiding police stations and political offices. U.S. and Iraqi forces were battling guerrillas in Mosul (search) hours later, and an American official acknowledged it could take "some time" to secure the city.

Smoke rose from several areas as U.S. warplanes streaked overhead. The attacks may have been launched in hopes of relieving pressure on the insurgent bastion of Fallujah (search), which U.S. and Iraqi forces stormed Monday.

Mosul authorities warned residents to stay away from the five major bridges across the Tigris River because of fighting in the area. Militants brandishing rocket-propelled grenades were in front of the Ibn Al-Atheer hospital in the city's Jammia district.

U.S. military spokeswoman Capt. Angela Bowman said some of the attacks on police stations overwhelmed "the capabilities of the existing police force" and that five police stations were ransacked.

"The insurgents continue to fire at the Iraqi National Guard and the multinational forces," Bowman said later. "The operations are still ongoing and probably will for some time until we fully secure the city."

She denied claims by some residents that parts of the city had fallen under insurgent control, saying guerrillas "have not taken any parts of the city."

Insurgents also attacked the headquarters of pro-American Patriotic Union of Kurdistan (search) party, forcing those inside to leave after overpowering the guards.

Residents saw masked gunmen roaming the streets, setting police cars on fire. The local television station in Mosul went off the air.

A curfew imposed this week remained in effect in the city about 225 miles north of Baghdad.

A U.S. statement said the local governor, Duraid Kashmoula, was "working with other regional governments to identify measures to restore and maintain a police presence in local police stations."

That suggested Iraqi authorities were planning to call in Iraqi reinforcements.