A homicide attacker detonated a car bomb Thursday outside the so-called Green Zone (search) that houses the U.S. headquarters in Baghdad, killing five Iraqi civilians and a U.S. soldier.

Twenty-five people, including two American soldiers, were injured in the blast. The bomb, hidden inside an orange-and-white Baghdad taxi, exploded outside of a three-foot-high concrete blast wall which protects a U.S. checkpoint.

"There was a long line of cars. Fortunately, the blast barriers worked in this case," said Col. John Murray of the U.S. Army's Texas-based 1st Cavalry Division (search).

A suicide bomber also died in the attack, the military said.

Hours later, a roadside bomb exploded on Saadoun Street, a busy commercial avenue on the east side of the Tigris River (search) near the Palestine and Sheraton hotels. Two Iraqis were injured.

Also Thursday, the U.S. command said that two U.S. soldiers were killed and two were wounded when a roadside bomb exploded in Baghdad. A statement said the explosion occurred just before midnight Wednesday but gave no further details.

The suicide attack came one day after U.S.-led forces launched their biggest assault yet against militiamen loyal to radical Shiite cleric Muqtada al-Sadr (search), raiding hideouts in several cities and clashing with gunmen. At least 15 Iraqis and a U.S. soldier were killed in Thursday fighting. In later overnight clashes, another 11 militiamen were killed, the U.S. military said.

Violence continued as special U.N. envoy Lakhdar Brahimi arrived in the Iraqi capital for intensive discussions aimed at forming a transitional government to take power June 30. Brahimi's arrival was confirmed by one member of the Iraqi Governing Council and another Iraqi official, both of whom spoke on condition of anonymity.

Moderate Shiites (search) tried to persuade al-Sadr to back away from his confrontation with the United States — a reflection of their growing concern.

The car bomb incinerated three vehicles, reducing them to hulks of twisted, charred metal. Another five cars were badly damaged, some turned on their side from the force of the blast.

The explosion was so strong that it hurled the engine of the car carrying the bomb some 15 feet from the site of the blast.

The U.S. soldier who died was the 21st U.S. serviceman killed in Iraq in May. The injured included two U.S. soldiers and three Iraqi policemen, the U.S. military said.

Shattered glass from nearby shops littered the area. A column of thick black smoke rose from the blast site and drifted across Baghdad. Residents living in homes as far as 100 yards away from the blast reported shattered widows and doors becoming unhinged.

On Jan. 17, a suicide truck bombing at a Green Zone gate in central Baghdad killed 24 people and wounded about 120. Three U.S. civilians and three U.S. soldiers were among the injured in the bombing at what U.S. soldiers call the "Assassins' Gate," an ornate gate leading to Saddam Hussein's former Republican Palace compound, now the headquarters of the U.S.-led occupation.

Blast walls and dirt-filled baskets were erected at that gate and other checkpoints in the Iraqi capital following the January blast. Murray said U.S. military checkpoints for car and pedestrian traffic remain "security worries." The checkpoints are used by foreigners who live and work inside the Green Zone, as well as thousands of Iraqis going to and from jobs inside the zone.

Also Thursday, gunmen assassinated the head of the local Agriculture Department in the northern city of Kirkuk in a drive-by shooting that also killed his driver and wounded his wife, police said.

Najib Mohammed, a Kurd, was riding in his car when the gunmen opened fire from another vehicle, Gen. Sherko Shakir said. Insurgents frequently target officials working for the U.S-backed Iraqi government, accusing them of collaborating with Americans.

In the south, heavy fighting against al-Sadr's militia came in the holy city of Karbala, where coalition forces raided a hotel, the local former Baath Party headquarters and the regional governor's office, where al-Sadr fighters had been stockpiling weapons, Brig. Gen. Mark Kimmitt said.

Troops came under fire in the overnight raid on the governor's office, Kimmitt said in Baghdad. He said 10 al-Sadr followers were killed. The U.S. soldier died when a dump truck tried to ram a checkpoint in Karbala, the military said.

Outside the city of Kufa, U.S. forces attacked a van where Iraqis were seen unloading weapons. The vehicle was destroyed and five Iraqis were killed, Kimmitt said.

In overnight fighting in Karbala, one militiaman was killed, a senior U.S. officer said, speaking on condition of anonymity.

In Baghdad's Sadr city, an overwhelmingly Shiite Muslim enclave, militiamen ambushed four U.S. patrols. Ten attackers were killed in retaliatory gunfire, the U.S. officer said. There were no reports of U.S. casualties.

In Najaf, U.S. troops battled al-Mahdi Army fighters outside a cemetery near the Imam Ali Shrine, Iraq's holiest Shiite site.

Iraqi Governing Council member Mohammed Bahr al-Ulloum delivered a message to al-Sadr from a group of influential Shiites calling on his militia to disarm and leave Najaf, council member Raja Habib Al-Khuzaai told The Associated Press.

The message from the group — made up of about 500 Shiites, including local council members, tribal officials and others — represented the most public effort by Shiite leaders to push al-Sadr into making concessions to end the standoff, which began when his militia launched an uprising in early April.