Three US soldiers were killed and six injured after a bombing caused a bridge across a highway near the Iraqi capital, Baghdad, to collapse.

The soldiers were manning a checkpoint on top of the bridge when a car bomb exploded beneath, the US military said.

The attack on Sunday was the latest in a series by Iraqi militants on bridges.

Meanwhile, the US says about 130 Sunni tribal leaders in the northern province of Salahuddin have agreed to take a more active role in fighting al-Qaeda.

A US military spokesman said the tribal leaders, who met in the town of Tikrit, had promised to do more to support the efforts of the security forces and the Iraqi government.

'Crushed'

Sunday's attack on the bridge over the main highway linking Baghdad to southern Iraq brought to at least 28 the number of US troops killed in Iraq so far this month.

A US military spokesman said the blast destroyed one of two sections of the "Checkpoint 20" bridge crossing the road, 10km (6 miles) east of Mahmudiya.

Lt Col Gary Bush said it appeared the suicide bomber had blown up his vehicle beside one of the overpass's supporting pillars.

The US Army checkpoint, including a tented rest area nearby, then fell into the debris. The checkpoint was reportedly open only to military traffic at the time.

Staff from a private security firm, Armor Group, who were in a convoy passing by the checkpoint at the time of the attack, worked with US troops to free the trapped soldiers.

"When that size blast went off, everyone was in shock," Jackie Smith of Armor Group said.

Donald Campbell, another member of the firm, said they had been able to pull a slab of concrete off one of the victims by using a Bradley armored vehicle.

"Another poor fellow looked crushed beneath a concrete slab," Mr Campbell added.

In other news, the UK's prime minister-designate, Gordon Brown, has arrived in Baghdad on a "fact-finding trip".

He will meet Iraqi politicians, and British and American troops.

"Gordon Brown is in Baghdad to gather facts that will inform decisions he needs to make about Iraq over the next couple of months," a spokesman for the British embassy in Baghdad said.

Mr Brown is making his first visit to Baghdad since it was confirmed he would succeed Tony Blair on 27 June.

PHOTO CAPTION

Collapsed bridge near Mahmudiya