A suicide bomber killed 12 people in the lobby of a busy Baghdad hotel on Monday, and the US military said six tribal leaders opposed to Al- Qaeda were among the dead.
It was one of four separate bombings across Iraq in which 40 people were killed.
Police said a bomber wearing a vest packed with explosives blew himself up after walking into the lobby of the Mansour Hotel, where Sunni Arab tribal leaders from western Anbar province had gathered for a meeting.
"According to initial reports, six sheikhs are among the dead," Lieutenant-Colonel Scott Bleichwehl, a US military spokesman in Baghdad, told Reuters.
Iraqiya state television said prominent tribal leader Fassal al-Igoud, a former Anbar governor and onetime deputy minister of agriculture, was among the dead. Iraqiya said one of its journalists was also killed.
Some Sunni tribal leaders in Anbar have joined forces to form US-backed provincial police units to fight against Sunni Islamist Al-Qaeda, prompting a bloody power struggle in the vast desert region.
One witness said the lobby of the high-rise hotel had been badly damaged by the blast just before noon (0800 GMT). "It was a huge explosion, the whole building shook for a few seconds," the witness said.
Another witness said he had seen seven charred bodies and pools of blood on the debris-littered lobby floor.
Police said 12 people had been killed and 18 wounded. Bleichwehl put the death toll at eight.
Visitors to the hotel are stopped at the main gate and only allowed inside by invitation. There is a metal detector inside the lobby, where visitors are first body-checked.
In the northern oil city of Baiji, 18 people were killed and another 40 wounded when a suicide bomber rammed a fuel tanker into protective walls outside a police headquarters, Baiji police Captain Ghazwan al-Janabi said.
In Mosul, about 120 km north of Baiji, a parked car bomb blew up in a residential area, killing three and wounding 40.
Police also said eight people died when a suicide car bomber struck outside the governor's office in the southern Shi'ite city of Hilla.
US and Iraqi officials blame most car bomb attacks in Iraq on Sunni Islamist al Qaeda. US forces have launched an offensive in regions around Baghdad to deny Al -Qaeda militants sanctuary in farmlands and towns from where they launch car bomb attacks.
Monday's blasts came after a relative lull in the number of such attacks in the past week. A car bomb killed 87 people at a Shi'ite mosque in central Baghdad on June 19.
Tens of thousands of US and Iraqi troops are taking part in "Operation Phantom Thunder", one of the biggest offensives by US and Iraqi forces against Al-Qaeda in Iraq since the US-led invasion to topple Saddam in March 2003.
While attempting to put a lid on the violence, the offensives are also an attempt to buy time for Prime Minister Nuri al-Maliki's Shi'ite-led government to reach a political accommodation with disaffected minority Sunni Arabs.