A suicide bomber blew up a truck on a bridge spanning the Tigris River in northern Baghdad on Thursday, partially destroying the bridge, killing 10 people and sending cars plunging into the waters below.
The morning rush hour attack came a day after US military for the first time charged that Shiite Iran was supporting Sunni extremist groups who are known to trigger such high-profile vehicle bombs against civilians and security forces.
The bombing ripped through the metal Al-Sarafiyah Bridge which connects the Shiite Al-Atafiyah neighbourhood on the western bank of the Tigris to the Sunni neighbourhood of Waziriyah on the east.
The bridge, one of the oldest in the capital, partially collapsed under the force of the blast, a security official said. Another official said a quarter of the bridge had been destroyed.
The metal construction was sliced in two after the attack, as river boat police and divers were seen searching for survivors, an agency photographer and a security official.
Ten people were reported killed and another 26 wounded, a security official said, adding that four cars tumbled down into the river below.
The riverside in central Baghdad was briefly filled with dust and haze, with vision temporarily obscured to the north after the attack on what is one of the oldest and tallest bridges in the Iraqi capital.
Although American and Iraqi officials have been upbeat about a reduction in execution-style killings in Baghdad since launching a huge security crackdown two months ago, they have admitted that car bombings remain a curse.
On Wednesday, US military spokesman Major General William Caldwell said civilian deaths in Baghdad had fallen by 26 per cent during the first quarter of 2007 compared with the previous quarter.
But he also revealed that the nationwide death toll of civilian and security forces had risen by 10 percent between February and March. According to Iraqi security officials in March alone more than 2,000 Iraqis were killed in the violence-wracked country, 15 per cent more than in February.
Amid increasing tensions between Iran and the United States, Caldwell said Wednesday that the military suspected Iranian agents were supporting Sunni extremist groups, something which US administration has never said before.
Washington has regularly charged that Shiite Iran was funding and training Iraq's Shiite militias but Wednesday's accusation that the former-foe of Iraq was also aiding Sunni groups was a first.
"We do have now some information that Iranian intelligence agencies have supported some Sunni extremist groups," Caldwell told reporters.
Caldwell displayed a cache of recently made explosives, bearing dates of 2005 and 2006, which he said were made in Iran and were found on Monday in a black Mercedes car in Baghdad's Sunni district of Jihad.
The US commander also accused the Iranians of training Iraqi groups on how to assemble explosively-formed projectiles -- a type of armour-piercing roadside bomb that has caused many coalition casualties.
"We know that training goes down there in Iran," said Caldwell, without giving further details or any source for the intelligence.
Such devices have been blamed for the deaths of at least 170 US service personnel since May 2004, largely in Baghdad and southern Shiite areas of Iraq.
But in a sign that the military is straining to meet its commitments, US Defence Secretary Robert Gates said soldiers will see their tours of duty in Iraq extended by three months.
"Effective immediately, active army units now in the Central Command area (Iraq and Afghanistan) and those headed there will deploy for not more than 15 months," Gates said -- an increase from the current one-year deployments.
Gates acknowledged that the US forces are stretched with the foreign deployments, however.
"There's no question about that," he said.
The new measure allows the army to maintain the surge in Iraq "probably at least" until April 2008, Gates said.
He added: "Whether it will be kept in place depends entirely on the conditions on the ground."