A fuel tanker rigged with explosives killed 37 people when it blew up near a Sunni mosque in western Iraq on Saturday, a day after the mosque's imam had criticized al Qaeda militants, police and residents said.

The bomb exploded in a market in the town of Habaniya in the restive province of Anbar, where U.S. forces are battling Sunni Arab insurgent groups, including al Qaeda.

Police said 64 people were wounded. An Interior Ministry source put the death toll at 31, with 67 wounded.

Local residents said the imam of the mosque had criticized Sunni al Qaeda during Friday prayers. Some Sunni tribal leaders in Anbar are leading a campaign to fight al Qaeda, which is deeply entrenched in the province.

Habaniya lies 85 km (50 miles) west of the Baghdad.

On Monday, two suicide bombers in nearby Ramadi killed 11 people when they targeted the house of Sattar al-Buzayi, who has led the anti-al Qaeda drive, which is backed by the Shi'ite-led government in Baghdad and the U.S. military.

Insurgents earlier stormed an Iraqi police checkpoint near Baghdad airport, killing eight policemen in a bold challenge to a U.S.-backed security crackdown in the capital aimed at halting sectarian violence.

President Bush is sending 21,500 extra troops to Iraq to help with the Baghdad crackdown. Most are heading for the capital, but 4,000 will also be sent to Anbar to try to quell the insurgency raging there.

CHECKPOINT ATTACK

Prime Minister Nuri al-Maliki expressed optimism about the 10-day old security plan, saying U.S. and Iraqi forces had killed around 400 suspected militants since it started.

But the attack on the police checkpoint in an area not far from the main U.S. military headquarters in Baghdad underlined the hurdles faced by Iraqi security forces who are often out- gunned by increasingly sophisticated insurgents.

"It was a brazen attack," said Captain Curtis Kellogg, a U.S. military spokesman. "It was definitely coordinated. We expect this type of thing to continue. They will try to test the Iraqi and U.S. security forces."

A statement from the U.S. military said eight to 10 gunmen attacked the checkpoint in two vehicles. Militants in the first one got out firing assault rifles and throwing hand grenades at the policemen.

The second vehicle was forced into a ditch where it was cordoned off on suspicion it could be a suicide car bomb.

Two militants were killed in the firefight. One was wearing a suicide vest, Kellogg said.

Maliki paid a visit on Saturday to the command center for the Baghdad operation and urged security forces not to be swayed by sectarian loyalties.

He told reporters 426 suspected militants had been detained in the crackdown "and around that number have been killed" since it was launched in mid-February. The campaign is regarded as the last chance to prevent all-out civil war.

The Shi'ite prime minister is under pressure from Washington to root out Shi'ite militias with as much determination as he has used against Sunni Arab insurgents.

But Friday's brief detention by U.S. forces of the son of Abdul-Aziz al-Hakim, one of Iraq's most powerful Shi'ite leaders, could strain the government's ties with Washington.

Hundreds of Iraqis took to the streets of Shi'ite towns on Saturday to protest at Friday's detention of Ammar al-Hakim.

There were no reports of violence. The U.S. military said Ammar al-Hakim was held on Friday because members of his convoy acted suspiciously at a border checkpoint while returning from Iran. He was released after several hours.

(Additional reporting by Ross Colvin, Mussab Al-Khairalla and Dean Yates)