A number of senior Sunni tribal leaders are among 12 people killed in a suicide bombing at a hotel in central Baghdad.
"According to initial reports, six sheikhs are among the dead," Lieutenant Colonel Scott Bleichwehl said.
The Sunni leaders, from Anbar province, were meeting at the hotel and were the target of the attack, police said.
Elsewhere in Iraq, at least 27 people died in a blast in Baiji, in the north, and eight people were killed in a blast in the southern city of Hilla.
Fifteen people were injured in the bombing at the Mansour Hotel, which is used by Westerners and Iraqi officials. The bomb exploded in the lobby.
Reports said Sunni tribal leader Fasal al-Khoud was among the dead.
Mr Khoud was one of the founders of the Anbar Salvation Council which has close contacts with the US-led coalition.
In recent months many of the tribes in Anbar have turned against al-Qaeda and related radical Sunni insurgent groups. This attack is likely to aggravate the rift between them, says the BBC's Jim Muir in Baghdad.
An Iraqi journalist, Rahim al-Maliki, who worked for al-Iraqiya TV, was also killed in the blast.
"It was a huge explosion, the whole building shook for a few seconds," one witness said.
A hotel worker, Saif al-Rubaie, 28, told the Associated Press news agency the casualties were Iraqis, mostly employees who were in the reception area.
The hotel is situated on the banks of the Tigris River, near Baghdad's Green Zone, and is often used by Iraqi officials visiting the capital.
A number of foreigners also live in the hotel.
In other attacks across the country, at least 27 people were killed, including 13 police, when an attacker rammed an oil tanker into the police headquarters in Baiji.
And at least eight people died when a bomb ripped into a crowd of police recruits outside the governor's office in Hilla, 100km (60 miles) south of Baghdad.
The attacks come despite a big security drive by US and Iraqi forces in Baghdad and Baquba, 60km (35 miles) north-east of the capital.
American generals have admitted that many senior al-Qaeda figures have escaped from the latest operations and that Iraqi forces are not up to the task of holding the ground that has been won.