BAGHDAD, Iraq - Iraqi security forces backed by U.S. troops detained almost 300 suspected insurgents overnight in the largest joint U.S.-Iraqi military offensive to date, the military said today.

The Baghdad offensive, dubbed Operation Squeeze Play, came as the American military announced that five U.S. soldiers were killed in northern Iraq on Sunday - four in separate roadside bomb attacks and one in a vehicle accident.

Two carloads of gunmen killed Maj. Gen. Wael al-Rubaei, a top national security official, and his driver in Baghdad's latest drive-by shooting. The killing came a day after another senior government official, Trade Ministry auditing office chief Ali Moussa, was shot dead - part of an ongoing terror campaign that has killed more than 550 people in less than one month.

In other violence, a suicide bomber killed five people and injured 13 when he drove an explosives-packed pickup truck into a crowd of people outside a municipal council office in Tuz Khormato, 50 miles south of the northern city of Kirkuk, said police commander Lt. Gen. Sarhat Qader.

In the former insurgent stronghold of Samarra 60 miles north of Baghdad, three suicide bombers tried to attack an American military base, injuring three U.S. soldiers, the military said.

Operation Squeeze Play, which began Sunday and was apparently winding down today, was centered on western Baghdad's Abu Ghraib district and targeted militants suspected of attacking the U.S. detention facility there and the road linking downtown to the international airport, the military said in a statement.

"This is the largest combined operation with Iraqi security forces to date," said U.S. military spokesman Lt. Col. Clifford Kent. "The Iraqi Security Forces have the lead in this operation while we perform shaping and supporting roles."

Seven Iraqi battalions backed by U.S. forces launched an offensive in the capital in an effort to stanch the violence that has killed more than 550 people in less than a month, targeting insurgents who have attacked the dangerous road to Baghdad's airport and Abu Ghraib prison.

"Iraqi army and ministry of interior forces worked very well together and demonstrated good, solid fundamental skills today," said Col. Mark A. Milley, commander of the 2nd Brigade Combat Team, 10th Mountain Division.

Aides to a radical anti-American Shiite cleric, meanwhile, sought to defuse tension between Sunnis and the majority Shiites after a recent series of sectarian killings. Sunnis are believed to make up the bulk of Iraq's raging insurgency.

Iraq's government took the diplomatic offensive, joining the United States in its oft-repeated demands that Syria close its porous border to foreign fighters.

Parliament was also expected to debate the release of Ghazi Hammud al-Obeidi, 65, one of the most-wanted officials from Saddam Hussein's former regime. Al-Obeidi was released last month apparently because he was purportedly terminally ill with stomach cancer.

Three Romanian journalists were expected to return home today. They and their Iraqi-American guide were released after being held captive for nearly two months. Iraqi insurgents had demanded Romania withdraw its soldiers from Iraq. Bucharest rejected the demand.