The operation in Babil province - an area notorious for kidnappings and ambushes and home to the fabled, ancient city of Babylon - follows last week's U.S.-Iraqi drive to oust insurgent forces from Samarra, about 60 miles north of Baghdad.

The 24th Marine Expeditionary Unit and Iraqi forces went into action after a string of bombings set off clashes Tuesday between U.S. troops and gunmen west of Baghdad and in the northern city of Mosul, and as the discovery of five beheaded bodies in two days indicated the pace of such grisly killings was also surging.

The Marines and Iraqis punched their way across the Euphrates River, rounded up 160 suspects, seized a suspected training camp and took control of a major bridge, the U.S. command said. The bridge, spanning the Euphrates, is believed to be a favored corridor linking insurgent areas around Baghdad, Fallujah and towns farther south.

The U.S. military lost significant areas of Iraq to insurgents after Saddam Hussein's ouster.

U.S. and Iraqi forces are trying to curb the mounting insurgency in order to hold national elections throughout the country in January. Some U.S. officials have expressed doubt that balloting will be possible in areas that have slipped from Iraqi government control.

Prime Minister Ayad Allawi said the tempo of attacks against insurgent strongholds would increase but acknowledged that the security challenge was a ''source of worry.''

''I don't want to deny the impact of security situation nor minimize the size of the challenges we face,'' Allawi said during a speech Tuesday in Baghdad. ''I believe that many of the Iraqi people agree with me that we should not let terrorist forces decide our agenda.''

''It's true that the security situation in which the country is living commands most of your attention and maybe your questions, too. It's true that it is a source of worry to many who are concerned about Iraq's future. But it's better than surrendering to the evil forces or giving in to their demands.''

As part of the campaign, Allawi's government has been negotiating with followers of renegade Shiite cleric Muqtada al-Sadr to halt weeks of fighting with U.S. troops in the Baghdad district of Sadr City. During his speech, Allawi said the two sides had reached the basis of a deal and talks were continuing.

However, clashes between al-Sadr's militia and U.S. troops continued despite the talks. Residents of the Shiite district reported hearing explosions Tuesday night.

U.S. troops also battled gunmen in an insurgent stronghold west of Baghdad and in Mosul after a series of bombings. At least five U.S. soldiers were wounded - underscoring the role of homemade explosives as the insurgent weapon of choice in a country awash in military ordnance.

In Ramadi, about 70 miles west of Baghdad, U.S. troops and insurgents exchanged gunfire after a car bomb exploded, police Capt. Nassir Hassan said. The U.S. military had no report of the incident, but an Associated Press photographer saw two dead Iraqis and four hurt at the scene.