BAGHDAD — Car bombs and gunmen struck new U.S. allies, police and civilians Sunday in northern Iraq, killing as many as 53 people. The spate of attacks came even as the American military released a captured diary and another document they say show al-Qaida in Iraq cracking under a Sunni revolt against its brutal tactics.

The violence coincided with a visit by Defense Secretary Robert Gates to Baghdad, where he warned that hard choices face Iraq's political leaders on how to stabilize the country despite promising new signs of progress toward reconciliation.

The deadliest bombing on Sunday was near Balad, 50 miles north of Baghdad, against a checkpoint manned jointly by Iraqi police and members of an awakening group.

Iraqi police said a suicide truck bomber targeted a checkpoint manned by U.S.-allied fighters and Iraqi police at the entrance of a bridge in the district of Yathrib on the outskirts of Balad. Security forces opened fire on the driver, but he managed to detonate his payload, devastating a nearby car market and other stores.

Police in the joint coordination center of the surrounding Salahuddin province and hospital officials said 34 people were killed and 37 others were wounded. Capt. Kadim Hamid said many residents in the predominantly Sunni area had removed victims directly from the site because they feared going to the hospital in Balad's mostly Shiite center.

The U.S. military put the casualty toll at 23 killed, 25 wounded and said a car bomb exploded near an Iraqi checkpoint in a market in Balad, but it did not confirm it was a suicide attack. U.S. and Iraqi forces had secured the area and the wounded had been evacuated to hospitals, according to a statement.

It was one of the worst bombings this year amid a recent lull in violence and underscored U.S. warnings that al-Qaida in Iraq remains a serious threat despite military offensives that have severely curtailed its operations.

The explosion came hours after suspected al-Qaida-linked insurgents stormed two villages near the Syrian border but were repelled by U.S.-allied fighters and Iraqi security forces in clashes that left at least 22 people dead.

Sheik Fawaz al-Jarba, the head of the Mosul anti-al-Qaida group, and other officials said the 22 killed included 10 militants and six members of the so-called awakening group in the area, as well as four women and two children.

The U.S. military in northern Iraq confirmed an attack on compound housing its Sunni allies against al-Qaida in Iraq near Sinjar, about 60 miles west of Mosul, saying five U.S.-allied fighters were killed, five wounded and 10 insurgents were killed.

Insurgents also attacked a group of civilians elsewhere in the northern Ninevah province on Sunday, killing two men and one child and wounding two other men, two women and two infants, according to the military.

Iraqi police also said four civilians were killed Sunday when a tanker truck laden with explosives blew up near an Iraqi army checkpoint on Mosul's southern outskirts.

Iraqi Prime Minister Nouri al-Maliki has promised a "decisive battle" against the terror network in Mosul but given no start date. The U.S. military has warned it will not be a swift strike, but rather a grinding campaign that will require more firepower.

An al-Qaida front group for northern Iraq warned last week in an Internet statement that it was launching its own campaign in Mosul and surrounding areas.

In all, 70 people were reported killed or found dead by police on Sunday, one of the highest nationwide death tolls in recent months. That figure included three policemen who perished in a suicide car bombing at a checkpoint in the Anbar city of Fallujah and 10 bullet-riddled bodies showing signs of torture.

Rear Adm. Gregory Smith, a U.S. military spokesman, said the documents released Sunday offered proof that al-Qaida in Iraq had been severely disrupted by the so-called awakening movement and changing U.S. tactics, but he stressed the terror network was by no means defeated.

The military said the two documents were discovered last year by American troops in November as the Sunni movement that began in Anbar province was spreading to Baghdad and surrounding areas.

One was a 39-page memo written by a mid- to high-level al-Qaida official with knowledge of the group's operations in Iraq's western Anbar province; the other a 16-page diary written by another group leader north of Baghdad.

The documents tell "narrow but compelling stories of the challenges al-Qaida in Iraq is facing," Smith told reporters in Baghdad. "This does not signal the end of al-Qaida in Iraq, but it is a contemporary account of the challenges posed to terrorists from the people of Iraq."

He said the documents are believed to be authentic because they contain details that only al-Qaida in Iraq leaders could know about battlefield movements and tactics. The U.S. military gave reporters partially redacted copies of the full diary but only four pages of the Anbar document, citing security reasons. Both were provided in the original Arabic and an English translation.

In the Anbar document, the author acknowledges a growing weariness among Sunni citizens of militants' presence and the U.S.-led crackdowns against them. He also expresses frustration with foreign fighters too eager to participate in suicide missions rather than continuing to fight.

"The Islamic State of Iraq is faced with an extraordinary crisis, especially in al-Anbar," the author wrote, referring to an umbrella group of insurgents led by al-Qaida.

Smith also quoted the document as lamenting the loss of "cities and afterward, villages," adding "we find ourselves in a wasteland desert."

It said U.S.-led forces had learned from their mistakes and improved security had made it harder to transport weapons and suicide belts and forced foreign fighters to go underground because of their distinctive dialects.

The military said the memo was believed to have been written last summer and was intended for the author's superiors.

The diary, seized by U.S. troops south of Balad, was written in autumn 2007 by Abu Tariq, who refers to himself as sector leader for al-Qaida in Iraq. Tariq wrote that he was once in charge of 600 fighters, but only 20 were left "after the tribes changed course" _ a reference to how many Sunni tribesmen have switched sides to fight alongside the Americans, Smith said.

The Sunni tribes' alliance with U.S. forces is credited with helping reduce violence across the country, along with an influx of some 30,000 American troops. A security crackdown that began in Baghdad and surrounding areas a year ago also has driven the militants north.

Mosul, 225 miles northwest of Baghdad and the capital of Ninevah province, is believed to be the last major urban stronghold for al-Qaida in Iraq.

"The diary shows that al-Qaida regards these volunteer citizen groups as a grave threat, and that terrorists are targeting them," Smith said.

In recent months, attacks on the Sunni volunteers have spiked while overall violence has steadily declined, he noted, adding that at least 77,500 volunteers have partnered with U.S. and Iraqi troops countrywide.

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