U.S. Marines and Iraqi soldiers have launched an offensive against insurgents in Iraq's western Anbar province, their sixth since the beginning of May, raiding suspected guerrilla strongholds outside Fallujah, the Marines announced Saturday.

The operation began Thursday in the village of Zaidon, about 20 miles southeast of Fallujah, and has continued in nearby towns and rural areas, the Marines said in a statement. The announcement of the sweep, called Operation Scimitar, was delayed until Saturday in a bid to catch insurgents by surprise, the statement said.

The Marines said 500 of their troops and about 100 Iraqi soldiers were attempting "to disrupt insurgent activity and help end their campaign of fear and intimidation against Iraqi citizens." No casualties were reported, and the Marines said they had detained 22 suspected insurgents.

The force deployed for the operation was about half as large as that used in other recent actions around Anbar, a province populated mostly by Sunni Muslims that has been a focal point of the two-year-old insurgency. The previous operations, between the beginning of May and early this month, generally involved about 1,000 Marines, soldiers and sailors, along with 100 Iraqi soldiers.

Through house-to-house searches, punctuated by street fighting and occasional airstrikes, the operations have sought to drive insurgents out of towns in Anbar where they have established footholds, the Marines have said. In addition, the U.S. and Iraqi forces have tried to cut off routes used by the insurgents to move men and supplies from Syria into Iraq.

Attacks by insurgents Friday and Saturday were largely concentrated in northern Iraq. In Baiji, 125 miles north of Baghdad, gunmen broke into a house Saturday morning and killed a family of four -- including two children, aged 2 and 5 -- according to the Reuters news agency. Near Mosul, 220 miles north of Baghdad, four travelers were pulled from their car and shot to death Friday, Reuters reported, and seven other Iraqis were killed in the city in a handful of attacks the same day.

Meanwhile, in southern Iraq, an area populated by Shiite Muslims that has been relatively peaceful for months, a radical Shiite cleric decreed that a petition be circulated demanding that U.S. and other foreign troops leave the country, the Associated Press reported. Moqtada Sadr, the cleric whose militia twice fought sustained battles against U.S. forces last year, said the petition should contain at least 1 million signatures.

Separately, Italy, one of several countries maintaining troops in the south, announced Friday that it planned to start withdrawing its forces in September. Prime Minister Silvio Berlusconi said 300 of the 3,000 Italian troops in Iraq would be pulled out in September if the security situation permitted.

Lance Cpl. Manuel Valle Jr. scales a wall in Ramadi, west of Baghdad, in a photo released by the Marines on Friday.