An undated photo provided by Thememoryhole.org shows US military personnel offloading coffins of US soldiers killed in Iraq at Dover Air Base in Delaware. US troop losses in Iraq topped 3,500 after a bridge bombing near Baghdad, as Britain's visiting future prime minister rejected domestic calls for a probe into the war's failings.[AFP]

Three US soldiers were killed and six wounded when their checkpoint was struck by a suicide car bomb, the military said, bringing the overall death toll to at least 3,501 since the March 2003 invasion, according to an AFP count based on Pentagon figures.

But despite the passing of yet another grim milestone for coalition forces, Britain's Gordon Brown said it was not the time for an inquiry into the war as called for by the Conservative opposition.

"The wrong time to even consider an inquiry is when you have got to give all your effort to supporting the troops on the ground," Brown told Sky News.

"There are times to consider these things, but the right thing to do at the moment is to give the full support and the full force of government behind the troops on the ground."

Monday's attack brought to 28 the number of US troops killed in Iraq so far this month, with more than 25,000 US troops wounded in combat since the invasion.

An interpreter was also wounded in the bombing, which destroyed part of a highway overpass in Mahmudiyah, a town south of Baghdad in a violent agricultural region known as an insurgent stronghold dubbed the "Triangle of Death".

The highway was partly blocked by debris from the collapsed bridge and the military said an engineering unit was dispatched with bulldozers and other heavy equipment to clear the road.

Last month, three US soldiers were snatched during an insurgent ambush on a small unit manning a temporary observation post near Mahmudiyah, during which four US soldiers and an Iraqi interpreter were killed.

An Al-Qaeda front group said a week ago it had killed the captured troops, an act Human Rights Watch said would constitute a violation of international humanitarian law and that those responsible would be guilty of war crimes.

Meanwhile, Brown, currently Britain's finance minister, held talks with Iraqi leaders including Prime Minister Nuri al-Maliki and President Jalal Talabani.

It was Brown's second visit to Iraq but his first since he was chosen to replace Blair in late June, and came amid mounting pressure at home over the war.

Brown has not indicated any plans to radically change Britain's policy on Iraq, but last month accepted that "mistakes" were made in the country.

On Monday, he vowed that Britain will continue to support Iraq, Talabani's office said.

"British minister of finance (Brown) praised the presidency and stressed the continuous support of his country to Iraq to achieve democracy, reconciliation and economic development," a statement from Talabani's office said.

"They also discussed the role of British forces to build and rehabilitate Iraqi military's abilities and participation in delivering peace and stability in southern Iraq" where British troops are based, the statement added.

About 150 British troops have been killed in Iraq. The government has pledged to withdraw this year about 1,600 troops from a force of 7,100 soldiers deployed in Iraq.

The BBC said the government would reject the opposition call for a probe, arguing that there have already been four inquiries and a new one will distract from British efforts to help stabilise the country.

"It's very important, for instance, to find whether there are lessons that need to be applied to Afghanistan from what has happened in Iraq for the last years," William Hague, the Conservative spokesman on foreign affairs, told BBC radio.

Britain has in the past year been increasing its deployment of soldiers to Afghanistan, where they are fighting a resurgent Taliban, while pledging to reduce troops in Iraq.

Violence continued in Iraq on Monday with at least five people killed, including a senior central bank official.

Khair al-Deen Sabri Ahmed, the bank's general manager in the northern Nineveh province, was shot dead by militants in the provincial capital of Mosul while on his way to work, said police Brigadier General Mohammed al-Waggaa.

His two bodyguards were also gunned down, and two other people were killed elsewhere.

A car bomb attack in the restive city of Baquba, northeast of Baghdad, on Monday led to the collapse of a key bridge, but no other details were immediately available, police said.

On the domestic political front, Iraq's parliament voted to oust its outspoken Sunni speaker Mahmud Mashhadani after he allegedly ordered his bodyguards to beat up a Shiite MP.