A suicide lorry bomber has attacked a bridge north of Baghdad, sending cars plunging into the water and killing at least 10 people, Iraqi police said.

The Thiraa Dijla bridge in Taji lies on the main road from Baghdad to Mosul.

Earlier, US troops said they had killed four gunmen in Baghdad, but local officials said at least three of the dead were civilians.

Separately, three Iraqi ministers whose party had been boycotting government meetings, attended the latest session.

The move comes as politicians from some of Iraq's main political parties are holding informal talks ahead of a summit called this week by Prime Minister Nouri Maliki to try to resolve their differences.

Also on Tuesday, the US military said four US soldiers had been killed in two separate attacks in Baghdad and the northern province of Nineveh on Monday.

'Bridge collapses'

The suicide lorry bomber attacked the Thiraa Dijla bridge at noon, police said.

He detonated his payload after passing through an Iraqi army checkpoint near the bridge in Taji, about 20km (12 miles) north of Baghdad.

The explosion killed at least 10 people and sent a number of vehicles plunging in the canal, which links the Tigris and Euphrates rivers.

The number of injured was not immediately known.

Reports say the bridge collapsed after the blast.

Government hopes

On the political front, the three minister from the mixed Sunni-Shia Iraqi National List of former Prime Minister Iyad Allawi unexpectedly attended a government session.

They were among five ministers who last week began the boycott of Mr Maliki's government over what they said was its failure to end sectarian violence.

Meanwhile, Foreign Minister Hoshyar Zebari said he hoped Iraq's fractured government could be rebuilt, and key legislation agreed, after this week's summit.

Most Sunni Arabs have withdrawn from the administration of Mr Maliki, whose cabinet is now narrowly based on Shia Muslim religious parties and Kurdish groups.

Washington has expressed its frustration with the slow pace of political reform, a central goal of its recent military build-up was to give Iraqi politicians the space to make real progress.

'Civilian deaths'

The US military said on Tuesday that its troops had killed four gunmen in Baghdad's Sadr City. But local officials said three civilians died in the raid.

They said one of the victims was a young girl sleeping with her family on the roof.

Later in the morning, angry mourners marched through the area with flag-draped coffins.

"The Americans raided our house from the roof... and killed my brother and my five-year-old niece," Ali Khamis Eidan, a policeman, told Reuters news agency.

The US military in Iraq denied killing any civilians in the raid.

Sadr City, home to about a third of Baghdad's six million people, has been a focus of American raids in recent weeks.

It is a stronghold in the capital of Shia militias largely loyal to cleric Moqtada Sadr.

PHOTO CAPTION

Grieving relative in Sadr City