BAGHDAD, IRAQ – More than 3,000 American and Iraqi forces launched a major operation yesterday against strongholds just south of Baghdad, their second mission in five days to wrest control from insurgents whose attacks threaten national elections seen as crucial to stabilizing this turbulent country.

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 American-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 yesterday between American troops and gunmen west of Baghdad and in the northern city of Mosul, and as the discovery of five beheaded bodies over 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 American 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 area in which they were operating was an estimated 40 to 45 miles northwest of ancient Babylon. The capital of Babil province is Hillah, about 65 miles south of Baghdad. The provincial capital sits near the Euphrates in a belt of rich agricultural land between that river and the nearby Tigris.

The American military lost significant areas of Iraq to insurgents after Saddam Hussein’s ouster. In remarks published yesterday, L. Paul Bremer, the top American civilian official in Iraq at the time, said “we never had enough troops on the ground.”

American and Iraqi forces are trying to curb the mounting insurgency in order to hold national elections throughout the country in January. Some American 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 the security situation nor minimize the size of the challenges we face,” Mr. Allawi said during a speech yesterday in Baghdad. “I believe that many of the Iraqi people agree with me that we should not let terrorist forces decide our agenda.”

Mr. Allawi added: “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, Mr. Allawi’s government has been negotiating with followers of renegade Shiite cleric Muqtada al-Sadr to halt weeks of fighting with American troops in the Baghdad district of Sadr City. During his speech, Mr. Allawi said the two sides had reached the basis of a deal and talks were continuing.

However, clashes between Mr. al-Sadr’s militia and American troops continued despite the talks. Residents of the Shiite district reported hearing explosions yesterday evening.

American troops also battled gunmen in an insurgent stronghold west of Baghdad and in Mosul after a series of bombings. At least five American 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, American troops and insurgents exchanged gunfire after a car bomb exploded, police Captain Nassir Hassan said. American military officials had no report of the incident, but an Associated Press photographer saw two dead Iraqis and four wounded at the scene. Later, a roadside bomb detonated as an American military convoy was passing near the Grand Mosque in the eastern section of Ramadi, wounding one soldier, said Marine spokesman First Lieutenant Lyle Gilbert. He said seven Iraqis were wounded.