A homicide bomber driving a truck loaded with TNT and toxic chlorine gas crashed into a police checkpoint in western Ramadi on Friday, killing at least 27 people and wounding dozens, police in the Anbar provincial capital said.

The bombing in Anbar province marked the ninth use of chlorine bombs in the sprawling, mainly desert territory that has been a stronghold of the Sunni insurgency.

Recently, however, many Anbar tribes have switched allegiance, with large numbers of military-age men joining the police force and Iraqi army in a bid to expel al-Qaida in Iraq fighters. Suicide bombings are an Al Qaeda trademark.

Police Maj. Mohammed Mahmoud al Nattah, member of the Anbar Salvation council, told state-run Iraqiya television the bomber hit a residential complex and dozens of wounded were taken to the Ramadi hospital.

Police opened fire as the homicide car bomber sped toward a checkpoint, three miles west of the city, according to police Col. Tariq al-Dulaimi. Nearby buildings were heavily damaged and police were searching the rubble for more victims.

South of Baghdad, Iraqi forces backed by American paratroopers swept into a troubled, predominantly Shiite city before dawn, and the U.S. military said as many as six militia fighters had been killed.

Lt. Col. Scott Bleichwehl, a U.S. military spokesman, said eight others were wounded and five detained. There were no reports of civilian casualties in the assault on Diwaniyah, he said.

Residents reported heavy fighting between the U.S. and Iraqi forces and gunmen of the Mahdi Army militia in the city, 80 miles south of Baghdad.

Dr. Hameed Jaafi, the director of Diwaniyah Health Directorate, said an American helicopter fired on a house in the Askari neighborhood, seriously wounding 12 people as the early morning assault began.

Bleichwehl said there were no U.S. air strikes either by helicopters or planes.

Also Thursday, the U.S. military confirmed an American helicopter carrying nine people had been downed south of Baghdad and that four were injured.

An Iraqi army official, speaking on condition of anonymity because of security concerns, said the helicopter crashed after coming under fire near the Sunni insurgent stronghold of Latifiyah, about 20 miles south of Baghdad. The U.S. military did not confirm that account.

It was the ninth U.S. helicopter to go down in Iraq this year. The U.S. military has studied new evasive techniques, fearing insurgents have acquired more sophisticated weapons or have figured out how to use their arms in new and effective ways.

Meanwhile, Prime Minister Nouri al-Maliki's government has decreed that men who held senior rank in the armed forces under Saddam Hussein be granted pensions and requested that those who had served in lower ranks rejoin the military as part of a sectarian reconciliation plan, according to a government statement issued Friday.

Al-Maliki's office said the decision was made during a Cabinet meeting late last month. It was not clear why the information was only released Friday.

Many former top intelligence, security and military officials are believed to have joined the Sunni insurgency after former U.S. administrator L. Paul Bremer disbanded Iraq's 350,000-member military on May 23, 2003, a month after the regime Saddam's regime was ousted.

The al-Maliki statement said that any former officer above the rank of major would be given a pension equal to that of officers now retiring. Former officers above major who wanted to rejoin the army were encouraged to check with the military command to learn if they were acceptable in the Iraqi army that is being rebuilt by American forces.

Those who had the rank of major or lower may voluntarily return to the army.

Lower ranking officiers and enlisted men with scientific or medical training would be given jobs in an appropriate government ministry, the statement said.