Baghdad, Iraq ? More than 3,000 U.S. and Iraqi forces launched a major operation Tuesday against insurgent strongholds just south of Baghdad, their second mission in five days to wrest control from militants 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 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 over two days indicated that the pace of such grisly killings was also surging.

The Marines and Iraqis punched their way across the Euphrates River, rounded up 30 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.

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.”

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

Locals check out a Bombed car after a passing U.S. convoy was targeted in Mosul, Iraq. Four U.S. soldiers were wounded and three Iraqis killed when U.S. troops opened fire after the explosion. The violence prompted a crackdown on insurgent strongholds Tuesday near Baghdad.