The US military also announced that six of its soldiers were killed in a bombing on Monday and another two lost their lives when a helicopter crashed, making May the bloodiest month for American forces this year.

In the latest kidnapping to blight the war-ravaged country, a security official said four Germans working for the Iraqi finance ministry were reported kidnapped in Baghdad by men wearing national police uniforms.

The men are computer science experts training civil servants at the ministry's information department on Palestine Street in the city centre when 40 police vehicles surrounded the building, he said.

"The ministry is checking the reports of the suspected kidnappings of German citizens in Iraq," a German foreign ministry spokesman in Berlin told AFP.

Britain's Foreign Office also said it was urgently checking the reports after the BBC said five Britons were believed to be among a group of Westerners kidnapped in the Iraqi capital.

"We're aware of reports that a group of Western nationals has been taken in Baghdad," a spokesman told AFP, adding the situation was unclear, with reports speaking of Germans, Britons or Americans.

"We're urgently looking into the reports."

The security official said four men were taken out at gunpoint and driven off by men wearing the new uniforms of the national police, a heavily-armed paramilitary unit under the interior ministry.

A Western security source, however, described the victims as one expert and three bodyguards.

In 2006, Baghdad went through a rash of kidnappings by large numbers of men wearing military-style uniforms, particularly those of the national police. The units were then issued new uniforms to distinguish them from criminals.

In February, a 61-year-old German woman married to an Iraqi, and her 20-year-old son were seized in Baghdad.

Thirty-three people were also killed in bomb attacks barely an hour apart in Baghdad, which is in the grip of a bombing campaign by insurgents bent on fomenting sectarian violence between the Iraq capital's rival Sunni and Shia factions and on undermining a 14-week-old US and Iraqi security plan.

A car bomb exploded outside a Shia prayer hall in southwestern Baghdad, killing 21 people and wounding 53 according to medical officials.

Survivors told Yarmuk hospital officials that the car exploded outside the al-Imma Husseiniya, a Shia prayer hall in the mixed Sunni-Shia neighbourhood of Amil.