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BAGHDAD - A suicide bomber struck the funeral of two Sunni tribesmen who joined forces against al-Qaida in Iraq, killing at least 50 people Thursday and reinforcing fears that insurgents are hitting back after American-led crackdowns.

The sudden spike in bloodshed this week adds to the other worries now piling up in Iraq: violent rivalries among Shiites and persistent cracks in the Iraqi security forces.

Violence across the country has declined since seven months ago, including dramatic suicide bombings like Thursday's funeral attack. American officials credit the change to the U.S. troop buildup and the rise of Sunni tribal groups known as Awakening Councils that have turned against al-Qaida-linked militants. A truce called last year by anti-U.S. Shiite cleric Muqtada al-Sadr has also helped.

But the new bloodshed highlights how fragile those gains are.

Thursday's attack happened in the town of Albu Mohammad, about 90 miles north of Baghdad. A suicide bomber dressed in traditional Arab robes passed unsearched by guards into a tent of mourners. The occasion was a funeral for two brothers who belonged to the local Awakening Council and who were killed in an attack a day earlier.

The bomber detonated explosives strapped to his body, killing at least 50 people and wounding dozens more, said police in the nearby city of Kirkuk.

"I first heard a thunderous explosion and when I turned my eyes to the tent I saw fire and smoke coming out," said Sheik Omar al-Azawi, an Awakening Council member who arrived at the funeral just before the blast. "Panicked people were jumping and running on all sides."

Insurgents also struck against Awakening Council members in Baghdad on Thursday. Two council members were gunned down in the Sunni district of Azamiyah. Hours later in the same area, five council members and a civilian were killed by a roadside bomb. And the head of the Awakening Council in the southern Baghdad area of Dora was killed by gunmen who sprayed his car with bullets, also wounding his son, police said.

The violence came two days after a string of suicide bombings in four cities of northern and central Iraq killed 60 people -- attacks that U.S. officials blamed on al-Qaida in Iraq.

There have been other sporadic bursts of dramatic attacks blamed on al-Qaida or other Sunni insurgents in past several months. It is unknown whether this week's violence signals that al-Qaida in Iraq has been able to reorganize after blows suffered from the U.S. troop surge and the Awakening Councils.

Death rates began declining significantly around September 2007 and reached an average low of 20 Iraqis killed per day in January, according to an Associated Press count. But since then, the levels have steadily climbed to an average of 41 reported killed per day last month.

U.S. military spokesman Maj. Gen. Kevin Bergner said such attacks do not detract from a markedly improved overall situation. "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 Wednesday.

The troubles on the Shiite front could be more dangerous. An offensive launched on March 25 in the southern city of Basra by Iraqi forces against Shiite militants -- particularly from al-Sadr's Mahdi Army -- touched off an uprising by Shiite militias across southern Iraq and in Baghdad's Sadr City.

Though the heaviest fighting of the operation has eased, clashes persist in Sadr City and the south, deeply straining the truce called by al-Sadr.

The Basra offensive also highlighted the continued weaknesses and divided loyalties that plague the Iraqi military, despite intensified U.S. efforts to train its forces.

The Iraqi government has acknowledged that during the Basra fighting, at least 1,000 Iraqi soldiers and police deserted or refused to fight because of intimidation from Shiite militiamen or loyalty to al-Sadr.

But details of the operation from an Iraqi army colonel in the Basra command center suggested the problems were even deeper.

The desertions came in the army's 14th Division, which is mainly composed of troops from Basra, the colonel said. Two brigades of about 600 troops each -- about 40 percent of the division's forces involved in the operation -- refused to fight, as did most of Basra province's 11,000 police forces.

The colonel, who spoke to the AP on Thursday on condition of anonymity in return for discussing the operation, said the soldiers' loyalties are sectarian and not to the nation.

Beyond the desertions, the offensive was hastily prepared and the Basra troops were poorly trained and badly equipped, the colonel said. "They are not professional enough, so they collapsed."

The troops lacked proper maps and communication equipment and were forced to rely on mobile phones to communicate. In contrast, the Mahdi Army fighters "had good, precise intelligence, better than ours" and more powerful weapons -- including anti-tank rockets and rocket-propelled grenades, he said. With only six tanks in all of Basra province, most Iraqi troops had only automatic weapons, he said.

With control of Basra's infrastructure largely divvied up between the city's Shiite factions, the Mahdi Army controlled the city's hospitals, meaning wounded soldiers could not be taken there, he said.

It was only because of the arrival of several military and police brigades from elsewhere in the country -- including special forces -- several days after the operation began that government forces were able to gain the upper hand in the city, the colonel said.

Prime Minister Nouri al-Maliki, who ordered the assault, has said it was a success. U.S. Ambassador Ryan Crocker had praised al-Maliki for his decision to strike at Shiite militias, but he acknowledged the operation ran into "a boatload of problems."