Over the past months, violence has dropped with the increase in U.S. troops and the growth of so-called Awakening Councils, groups of Sunni tribesmen who joined American forces in fighting al-Qaida-linked militants.
Thursday's attack took place in the town of Albu Mohammed, 90 miles north of Baghdad, during the funeral of two brothers who belonged to the local Awakening Council. The brothers were slain a day earlier, police said.
The suicide bomber walked into a tent crowded with mourners in the village and detonated explosives strapped to his body, police in the nearby city of Kirkuk said.
One witness, Sheik Omar al-Azawi, was just pulling up at the tent in his car when the blast went off.
"I first heard a thunderous explosion and when I turned my eyes to the tent I saw fire and smoke coming out," al-Azawi, 51, told The Associated Press in a telephone interview.
"Panicked people were jumping and running in all sides and then we started to evacuate those who were killed and wounded in our private cars until police and medical teams arrived," he said.
At least 50 people were killed and 20 in the blast, the police officials said, speaking on condition of anonymity because they are not authorized to talk to the media. The blast was the single deadliest attack since March 6, when a bombing in central Baghdad killed 68.
Thursday's attack came on the heels of a string of suicide attacks on Tuesday that killed 60 people in four major cities in central and northern Iraq.
The U.S. military has touted the relative calm in Sunni areas as a major success of the troop surge and the strategy of encouraging Awakening Councils and other Sunnis -- some former insurgents -- to turn against al-Qaida.
The new Sunni violence comes as fighting has increased between U.S.-Iraqi forces and Shiite militiamen, particularly members of anti-U.S. cleric Muqtada al-Sadr's Mahdi Army.
On Wednesday, fresh clashes broke out in the Baghdad Mahdi Army stronghold of Sadr City between U.S.-backed Iraqi troops and Shiite militiamen, leaving two men dead and 18 people wounded, police said.
In the southern city of Basra, a U.S. drone killed four militants when it fired rockets at militiamen who attacked an Iraqi army patrol.
An offensive launched on March 25 by Iraqi forces against Shiite militants in Basra touched off an uprising by Shiite militias across southern Iraq and in Sadr City.
U.S. military spokesman Maj. Gen. Kevin Bergner said Wednesday that despite this week's violence, the overall situation in Iraq has markedly improved over the past year.
"We have said all along that there will be variants in which we will see al-Qaida and other groups seek to reassert themselves," Bergner said.
On Wednesday, the Iraqi government said it was replacing two senior military commanders overseeing operations in Basra, Iraq's second-largest city.
Officials insisted the two -- security army commander Lt. Gen. Mohan al-Fireji and police chief Maj. Gen. Abdul-Jalil Khalaf -- had not been fired but were being reassigned to positions in Baghdad after their assignments ended.