A homicide bomber struck the funeral of two anti-Al Qaeda Sunni tribesmen in a town north of Baghdad on Thursday, killing at least 50 people and wounding dozens, police said.

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 and former insurgents who have joined American forces in fighting Al Qaeda-linked militants.

Thursday's attack took place in the town of Albu Mohammed about 150 kilometers (90 miles) north of Baghdad, during the funeral of two brothers who belonged to the local Awakening Council and had been killed in an attack a day earlier, police said.

The homicide 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.

The head of the local Awakening Council, 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.

He said the bomber, believed in his late 50s, was dressed in traditional Arab robes and that guards in charge of searching mourners allowed him in without a search.

At least 50 people were killed and 50 injured 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 homicide 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 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.

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.

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 Prime Minister Nouri 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.