The military said in a statement that some 1,500 U.S. Marines, 500 U.S. soldiers and 500 Iraqi soldiers were participating in Operation Iron Hammer near the town of Hit, 140 kilometers west of Baghdad.
The statement said the forces would concentrate efforts in the Hai Al Becker region.
In the central town of Al-Baquba, Iraqi police said unidentified gunmen opened fire on a minibus, killing nine construction workers.
Meanwhile, five Westerners remain in captivity in Iraq.
In a videotape, kidnappers say they will kill the German and her Iraqi companion unless Germany stops all cooperation with Iraq.
German Chancellor Angela Merkel today warned Germany would not give in to what she called "blackmail.
"This government, this parliament, will not let itself be blackmailed, that is clear," Merkel told the German parliament. "What is also clear is that all the efforts of the government are diverted to this situation and to protecting the lives of [German archaeologist] Susanne Osthoff and her Iraqi companion and we are working for their release."
(compiled from agency reports)