U.S.-backed Iraqi forces killed about 250 militants in a daylong battle near Najaf aimed at preventing attacks on religious pilgrims during major Shiite rituals, Iraqi authorities said.

The U.S. military confirmed that a helicopter went down Sunday during combat operations near Najaf and the two crew members were killed.

Col. Ali Nomas, spokesman for Iraqi security forces in Najaf, said more than 250 corpses had been found at the scene. Army Maj. Gen. Othman al-Ghanemi also spoke of 250 dead but said an exact number would be released Monday.

He said 10 gunmen had been captured, including one Sudanese.

Provincial officials said the gunmen planned to attack Shiite pilgrims and clerics during ceremonies marking Ashoura, which commemorates the 7th century death of Imam Hussein. His death began the schism between Sunnis and Shiites.

Complete coverage is available in FOXNews.com's Iraq Center.