U.S. forces mounted their biggest offensive in a year against Sunni Arab insurgents in western Iraq on Saturday, saying they would make the lawless area on the Syrian border safe for voters in next month's election, a report carried by Reuters said. An assassination attempt in Baghdad on a Sunni leader whose group accused the Americans and the Shi'ite-led government of killing civilians in such operations added to sectarian tensions that are driving the campaign for the Dec. 15 ballot, in which Sunnis are expected to vote in large numbers for the first time. "The government is unable to control the situation," fellow Sunni politician Hussein al-Falluji said after Fakhri al-Qaisi was hit by five bullets as he drove alone in the capital. Qaisi had said he feared for his life from Shi'ite "death squads", as well as Sunni insurgents opposed to his role in the U.S.-backed political process; he was in a critical condition. Some 2,500 U.S. troops and 1,000 local Iraqis met "sporadic" resistance, a U.S. Marines statement said, when they advanced through the streets of Qusayba on the Syrian border at the start of Operation Steel Curtain against foreign al Qaeda fighters. --More 2328 Local Time 2028 GMT