A blast ripped through a canteen in the Iraqi parliament complex inside Baghdad's heavily-fortified Green Zone on Thursday, killing two lawmakers, state television said.
The attack marks a major breach of security in the Green Zone, which is also home to the Iraqi government and foreign embassies and is the most heavily guarded site in Iraq.
State television said two lawmakers were killed and another 10 wounded.
The explosion occurred towards the end of lunch, when some lawmakers were finishing their meals and others were speaking to journalists, a security official said.
The blast occurred just hours after a suicide bomber blew up a truck on a major bridge across the Tigris River in Baghdad, killing 10 people and sending cars plunging from the wrecked structure into the waters below.
Although American and Iraqi officials have been upbeat about a reduction in execution-style killings since launching a huge security crackdown nearly two months ago in Baghdad, they have admitted car bombings remain a curse.
On Wednesday, the US military for the first time charged that Shiite Iran was supporting Sunni extremist groups which are known to trigger such high-profile vehicle bombs against civilians and security forces.
A large part of the Al-Sarafiyah Bridge, one of the oldest in the Iraqi capital, collapsed under the force of Thursday's blast.
River police were seen racing to the scene on patrol boats and divers donned oxygen cylinders to search the murkey waters for survivors.
Ten people were reported killed and 26 wounded, a security official said, with two officials saying four cars tumbled off the bridge which connects the Shiite Al-Atafiyah neighbourhood on the western bank of the Tigris to the Sunni district of Waziriyah on the east.
A witness, who gave his name only as Jawad, told an agency photographer that he was on the bridge trying to fix a puncture to his vehicle loaded with cooking gas when he saw a man park a truck nearby and run off.
"I saw the man get out of the vehicle and run away towards Al-Waziriyah. I was astonished and told an army patrol," he said.
The witness said Iraqi soldiers sealed off the bridge to traffic before the truck exploded, perhaps explaining why the death toll was not higher.
Security officials, however, said it was a suicide truck bomb.
On Wednesday, US military spokesman Major General William Caldwell admitted that the overall Iraqi death toll had risen by 10 per cent in March alone, 15 percent more than in February.
In a sign that the American military is straining to meet its commitments, US Defence Secretary Robert Gates has 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 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
Amid rising tensions between Iran and the United States, Caldwell said 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 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 scores of coalition casualties.