A U.S. airstrike has killed a local leader of the terror group ISIS in the Fallujah, Iraq, a spokesman for an American-led coalition fighting the group said. U.S. Army Col. Steve Warren said the strike Wednesday on ISIS' headquarters in Fallujah killed local commander Maer a-Bilawi. The attack was one of more than 20 airstrikes that killed 57 ISIS fighters over the past few days, Warren said. Thousands of Iraqi military, police and Sunni tribal fighters are poised to launch an assault on Fallujah to retake the largely Sunni city from ISIS control. Coalition aircraft have dropped leaflets instructing the 50,000 civilians who remain in Fallujah to cover their rooftops with white sheets to avoid airstrikes. The majority Sunni capital was Iraq's first city to fall to ISIS in January 2014. Retaking it promises to be a major challenge for the country's beleaguered security forces. Fallujah was a center of the insurgency led by the ISIS's predecessor, al Qaeda in Iraq, before the U.S. withdrawal in 2011. More than 100 U.S. troops died and another 1,000 were wounded fighting insurgents in house-to-house battles in 2004. This story originally appeared on NBCNews.com.