The troops were killed on Monday near Haditha, a town on the Euphrates river 200 km northwest of Baghdad, said Lieutenant Colonel Steve Boylan, a US military spokesman.

It was not immediately clear if they were killed in a single attack or if they died in a clashes with insurgents.

There have been frequent insurgent attacks in the area around Haditha in recent months. US forces have launched at least two major offensives to try to quell insurgency in the region, one of the most violent in the country.

At least 23 people were killed in rebel attacks across Iraq yesterday as members of parliament discussed issues delaying the completion of the war-torn country's new constitution.

A powerful blast shook central Baghdad when a suicide car bomber blew himself up close to a US military convoy, killing four people and wounding 23 others, including four women, medics said.

"We have received four bodies and 23 other people who are wounded from the car bomb," said a doctor at Ib-Nafis hospital.

One US humvee was set ablaze and 14 other vehicles were damaged by the blast, which occurred at around 1:00 pm (0900 GMT), an AFP correspondent reported.

There were no immediate reports of US casualties.

"We were 20 meters (yards) away when the car bomb went off and me and my team carried 15 wounded civilians," said policeman Hussein al-Musawi, his shirt soaked with blood.

The latest car bombing came as parliament worked on a draft constitution that MPs have vowed will be ready by August 15, on time for a scheduled mid-October referendum.

US officials have pressured Iraqi leaders to complete the document on time and keep to the timetable, amid fears a delay could undermine public confidence in the political process and play into the hands of insurgents.