BAGHDAD, July 24: A suicide bomber in a flatbed truck laden with 220kgs of explosives killed up to 40 people outside a Baghdad police station on Sunday, the US military said, citing Iraqi police reports.

But Iraqi police and interior ministry sources told Reuters 22 people had been killed and 25 wounded.

The bloodshed came amid growing tensions over a committee drafting a constitution that is seen as a vital mechanism for drawing Arab Sunnis, who form the bulk of the Iraqi insurgency, into a peaceful political process.

Television pictures showed a deep crater in the road as ambulances and fire-fighters attended the scene. The wreckage of a vehicle smouldered more than an hour after the blast.

The attack was the deadliest since a suicide bomber blew himself up next to a fuel truck on July 16, causing a huge conflagration that killed 98 people in a town south of Baghdad.

“This is a very cowardly act carried out by criminals, not Mujahideen,” said a police major who gave his name as Kasim.

Militants have stepped up suicide attacks in a campaign to topple the US-backed Iraqi government. There have been more than 20 blasts in the past 10 days alone.

“The car bomber made a deliberate decision to attack the Iraqi police station,” said Major Russell Goemaere, spokesman for the US 2nd Brigade Combat Team.

“The terrorists undoubtedly see the improved Iraqi police services as a threat to their operations.”

Despite that interpretation, a senior US military official in Washington offered a more cautious view of the ability of Iraqi forces in a report released last week.

“Only a small number of Iraqi security forces are taking on the insurgents and terrorists by themselves,” said an assessment provided to the US Senate by Marine Corps General Peter Pace, vice chairman of the US military’s Joint Chiefs of Staff.

Overwhelmed by violence, the Iraqi government hopes to give Sunni Arabs more power in a bid to defuse the insurgency and ease sectarian tensions.

But Sunni officials on the constitution-drafting committee, who walked out after a Sunni member and an observer were shot dead last week, say they will not return unless their demands, including an international investigation, are met.

In Amman, Iraqi Foreign Minister Hoshiyar Zebari said Iraq would finish writing the constitution and hold elections at the end of the year, even if the once dominant Sunni minority continued to boycott the process.

“It is in the interest of the Sunnis to participate without making excuses,” Zebari told Reuters.

“If they do not take part, the constitution will not reflect their hopes and ambitions and the process will not stop. There is a timetable and Iraq has to honour international commitments according to UN resolutions,” said Zebari, who chaired a meeting of senior Iraqi diplomats in the Jordanian capital.

Signs that Sunnis might turn to politics, rather than violence, had encouraged members of the government but problems with the constitution committee have raised fresh concerns.

Asked whether Sunnis would accept an Iraqi government investigation into the killing of the Sunnis on the constitutional committee, Mutlaq said: “We believe that the government is not qualified to undertake an investigation that will lead to results that will expose the crime.

“We believe that the nature of the crime points to the involvement of militias inside the state in this crime. The accused cannot be the judge.”

Sunnis have accused the government of sanctioning Shi’ite hit squads, a charge denied by Iraqi officials.

Police said gunmen killed a Shia family of four near the southern town of Hilla. Such attacks have fuelled concerns that sectarian tensions could spark a civil war.

SAUDI DIPLOMAT: In a related development, Prince Turki al-Faisal, Saudi Arabia’s incoming US ambassador, said on Sunday that Iraq’s government needed to boost its border security in order to stop terrorists entering its territory.