An insurgent squad shot dead the commander of Iraq's new counter-insurgency headquarters as he drove to work Monday in Baghdad, as US and Iraqi troops conducted a massive sweep for insurgents.

Suicide bombers attacked a US base in central Iraq and a town hall in the north of the country, killing five people, in the latest violence to mar the country's transition.

Three Romanian journalists, released by hostage-takers after two months of captivity, flew home as another insurgent group, led by Al-Qaeda frontman in Iraq Abu Musab al-Zarqawi, claimed to have executed an Iraqi-American businessman kidnapped four days previously in the capital.

Five US soldiers were killed in Iraq on Sunday, the US military said Monday, including three in attacks in the northern town of Mosul, one in a bombing near Tikrit, central Iraq, and one in a road accident near Kirkurk, further north.

This brings the total number of US service personnel who have died in Iraq since the March 2003 invasion to 1,626, according to a tally based on Pentagon figures.

Major General Wael Rubaye, the new commander of a special operations room recently set up by the ministry for national security to coordinate the fight against insurgents, and his driver were shot dead by insurgents in the capital early Monday, the cabinet office said in a statement.

Insurgents killer squads have repeatedly targetted senior police and army officers, along with civil servants, in the capital over the past weeks.

More than 15,000 soldiers from four Iraqi army battalions and three special police commando battalions, backed by US troops, arrested 285 suspects in the first 24 hours of operations as they swept areas west of Baghdad in an attempt to crack down on insurgents and car bomb-making operations.

"Operation Squeeze Play" is "the largest combined operation with Iraqi Security Forces to date," according to Lieutenant-Colonel Clifford Kent, a coalition spokesman.

The area swept by US and Iraqi forces since Sunday includes neighbourhoods where many of the attacks carried out daily on the airport road are thought to originate.

Convoys carrying US troops, private security guards, foreign contractors and journalists are frequently hit on the airport road, a 12-kilometre (seven-mile) stretch nicknamed the "Death Strip".

The month of May has been one of the bloodiest since the March 2003 US-led invasion of Iraq, with relentless car bombs and other attacks killing more than 500 people.

In the latest wave of bombings, two men blew themselves up after attempting to ram their cars packed with explosives into a US army base in Samarra, 125 kilometres (75 miles) north of Baghdad, the US military said.

Three US soldiers were lightly injured by the twin blasts.

As other soldiers emerged in response to attack, a third suicide bomber, wearing a vest packed with explosives, ran up to the entrance of the compound and blew himself up after being shot by soldiers. The blast killed the bomber but caused no further injuries or damage, the US army said.

In a separate attack in Samarra Monday morning, two Iraqi soldiers were killed and one was wounded when insurgents fired 10 mortar bombs at a joint army-police base, said army captain Salam Hadi.

In another car bombing incident, five people were killed and 19 wounded when a driver in a pick-up truck blew himself up outside the town hall in the northern town of Tuz Khurmatu, 70 kilometres (45 miles) south of Kirkuk, said police captain Imad Abdallah.

Meanwhile, three Romanian journalists captured in Iraq nearly two months ago have been set free and were expected to fly home Monday.

Marie Jeanne Ion, a journalist with Prima TV and her cameraman Sorin Miscoci, along with Eduard Ohanesian, a correspondent for the Romania Libera newspaper, and Mohamed Munaf, their guide were freed on Sunday, officials said in Bucharest, adding that all were in good health.

Romania made no foreign policy concessions and paid no ransom to secure the release of the hostages, Romanian President Traian Basescu said Sunday.

The kidnappers had earlier issued an ultimatum calling on Bucharest to withdraw its 860 troops from Iraq or see the hostages executed.

The group of Al-Qaeda frontman in Iraq Abu Musab al-Zarqawi Sunday claimed to have executed an Iraqi-American businessman, kidnapped four days earlier in Baghdad.

Their statement was posted on the Internet and accompanied by pictures of an Illinois driving licence identifying the purported victim as Neenus Y. Khoshaba, a US national born November 27, 1948.

According to a high-ranking official with Iraq's Assyrian Democratic party, Khoshaba was a US-Iraqi businessman from an Assyrian family.

"I can't confirm his death but I can identify the hostage. Mr Khoshaba was a US-Iraqi businessman based in Chicago who moved back to Iraq after the war in 2003," the official said on condition of anonymity.

"He was apparently tricked by a group of people posing as representatives from the oil ministry, who told him that they were looking for someone with a dual nationality and had business opportunities to offer him," the official added.