The blast was the latest this week to break a period of relative calm in Sunni areas, raising concerns that Sunni insurgents are reorganizing.
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 have joined American forces in fighting al Qaeda-linked militants.
The suicide bomber detonated his explosives in the town of Albu Mohammed, about 90 miles north of Baghdad, during the funeral of two brothers who belonged to the local Awakening Council who were slain a day earlier, police in the nearby city of Kirkuk said.
At least 50 people were killed and 20 were wounded 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 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 attack came a day after Prime Minister Nourithat his government would conquer al Qaeda in Iraq. "We are today more confident than any time before that we are close to the point where we can declare victory against al Qaeda... and its allies," he said in an address to the European Parliament in Brussels, Belgium.
Dozens of Iraqis have been killed this week by bomb blasts. Just one day before al-Maliki's bold comments, nearly 60 people died in a series of blasts in four cities in northern and central Iraq. Those attacks were blamed on al Qaeda.
struck directly at U.S. claims that the Sunni insurgency is waning and being replaced by Shiite militia violence as a major threat.
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 Qaeda.
U.S. military spokesman Maj. Gen. Kevin Bergner said Wednesday that despite this week's stepped-up 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 Qaeda and other groups seek to reassert themselves," Bergner said.
But 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.
In other developments:
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.
The two Iraqi officers will be replaced by new security commander Maj. Gen. Mohammed Jawad Huwaidi and new police chief is Maj. Gen. Adil Daham, officials said.
U.S. officials have praised al-Maliki for the determination he showed in confronting the militias, but they have also said the Basra operation was hastily arranged and badly executed. Critics said it highlighted the Iraqi army's poor leadership and the low morale among its rank and file after some 1,000 troops deserted or refused to fight in Basra.