A suicide truck bomber flattened a Shiite mosque Friday in northern Iraq, and roadside bombs struck Shiite pilgrims in Baghdad, as at least 51 people were killed and scores wounded nationwide.

It was the second-deadliest day since U.S. forces turned over urban security to the Iraqis more than a month ago, raising fears that Sunni insurgents are intensifying a campaign to re-ignite sectarian violence that pushed the country to the brink of civil war.

The blast in a northern suburb of Mosul reduced the mosque and several nearby houses to rubble, leaving scores of worshippers and neighbors trapped underneath. Rescue crews and ordinary citizens joined forces to pull bodies from the debris and search for survivors.

At least 38 people were killed and some 200 wounded, according to police.

The attack targeted a mosque used by members of the minority Shiite Turkomen community in the tense northern city, which the U.S. military has dubbed the last urban stronghold of Al Qaeda in Iraq. Suicide car bombings are the signature style of attack used by the terror network.

Witnesses said the explosives apparently were hidden in white bags usually used to transport grain and casualties were high because the blast struck as funeral services were being held along with Friday prayers.

"I can say that every house in this humble, small village was affected and even the glass of far away cars was shattered due to the force of the blast," said Jaafar Mohammad, whose uncle was killed. "The truck was loaded with huge quantities of explosives."

The governor of the surrounding Ninevah province, Atheel al-Nujaifi, said many of the wounded were in critical condition. He blamed Iraqi security forces for failing to secure the area on the northern outskirts of Mosul.

Maj. Derrick Cheng, a spokesman for U.S. forces in northern Iraq, said the attack was likely part of a strategy to discredit the provincial government and local security forces as well as fuel ethnic and sectarian tensions.

"These attacks can be seen as an attempt to feed any divide that currently exists or attempt to create one," he said in an e-mail.

Roadside bombs also targeted Shiite pilgrims returning from the holy city of Karbala, where hundreds of thousands of followers converged to celebrate the birth of Mohammed al-Mahdi, the 12th Shiite imam, who disappeared in the ninth century.

Devout Shiites call him the Hidden Imam and believe he will return to restore peace and harmony. The ceremonies concluded early Friday morning. Shiite pilgrimages have frequently been targeted by bombers and gunmen since resuming in force after the Sunni-dominated regime of Saddam Hussein was ousted in the 2003 U.S.-led invasion.

The first bomb targeted a minibus ferrying pilgrims back to Baghdad's main Shiite district of Sadr City, killing at least four people and wounding eight, according to police and hospital officials.

Sattar Jabbar, a 29-year-old laborer who was sitting on top of the bus, said those killed were sitting inside to escape the searing summer heat as they returned from Karbala, 50 miles south of Baghdad.

"We were filled with joy and spiritual happiness after the pilgrimage ... though we were also tired because of the crowds and traffic," he said. "I blame myself and feel guilty because a friend of mine was trying to climb to the roof to join us, but unfortunately I suggested he take a seat inside the bus because of the heat and that caused his death."

Two nearly simultaneous bombs exploded later near the Shaab stadium in eastern Baghdad killed three pilgrims and wounded 13 others as they were walking home to Sadr City, said another police official.

The blasts came a week after a string of bombings targeting Shiite mosques in the Baghdad area that killed at least 29 people.

Hours later, an explosives-laden motorcycle blew up as a police patrol passed by near people lined up to buy bread at a bakery in a mainly Sunni neighborhood in Baghdad, killing at least six people — including three policemen — and wounding 30, officials said.

U.S. commanders have said they were pleased with the progress since U.S. combat troops pulled back from cities on June 30 as part of a withdrawal plan that would see all American forces out of Iraq by the end of 2011.

But Iraqis continue to face daily attacks, including spikes in bombings followed by periods of relative calm.

On July 9, a total of 56 people were killed in bombings in the northern, mainly Turkomen city of Tal Afar and Baghdad — the deadliest day since the handover.

U.S. officials have repeatedly called the security gains fragile and cautioned that a waning insurgency still has the ability to pull off high profile bombings. They urged Shiites to show restraint to prevent a return to the retaliatory attacks that caused sectarian violence to spike after the Feb. 22, 2006, bombing of a revered Shiite mosque in Samarra.

Earlier this week, Ad Melkert, the new U.N. special envoy for Iraq, welcomed a new sense of optimism since the Iraqis took over security of the cities but said "reality is still tainted by an unacceptably high level of indiscriminate attacks on civilians."

The Security Council voted unanimously on Friday to extend the U.N. civilian mission in Iraq, commending the country's efforts to strengthen democracy but stressing the need to improve security and human rights.