A car bomb exploded Sunday near an Iraqi checkpoint in a market north of Baghdad, killing at least 23 civilians and wounding 25, the U.S. military said.
The violence was among a series of attacks in northern Iraq.
The military said U.S. and Iraqi forces had secured the area and the wounded had been evacuated to hospitals.
Iraqi police said earlier that a suicide car bomber targeted U.S.-allied fighters and Iraqi security forces at a checkpoint in Yathrib, near Balad, 50 miles north of Baghdad.
Police and members of an anti-al Qaeda group opened fire as the attacker sped toward a joint checkpoint, but he managed to detonate his explosives near some shops about 20 yards away, according to police at the Salahuddin provincial coordination center. Police said eight civilians were killed and 20 wounded.
The differing casualty figures and details could not immediately be reconciled.
Meanwhile, Iraqi police said four civilians were killed Sunday when a tanker truck exploded near an Iraqi Army checkpoint south of Mosul.
In an attempt to avert a suicide attack, Iraqi soldiers opened fire on the tanker about 30 yards from their post, an officer said on condition of anonymity because he was not authorized to speak to media. The tanker exploded, killing four civilians and damaging six cars nearby, he said.
The blast also damaged a gas station near the checkpoint on the outskirts of Mosul, which lies 225 miles northwest of Baghdad.
The explosions came hours after suspected al Qaeda-linked insurgents stormed two villages in northwestern Iraq but were repelled by U.S.-allied fighters and Iraqi security forces in clashes that left at least 22 people dead, according to Sheik Fawaz al-Jarba, a Sunni lawmaker and the head of the anti-al Qaeda group in Mosul.
The attack began about 5 a.m. when about 25 carloads of heavily armed gunmen, drove into the villages of Khams Tlol (Five Hills) and al-Madina, about 50 miles west of Mosul, said Sheik Fawaz al-Jarba, a Sunni lawmaker and the head of the anti-al Qaeda group in Mosul.
He said villagers fought back against the militants, who were wielding rocket-propelled grenades, mortars and assault rifles, prompting clashes that lasted about five hours.
An Iraqi army officer in Mosul, who spoke on condition of anonymity because he wasn't authorized to disclose the information, confirmed the attack and said the fighting ended after Iraqi soldiers joined the battle.
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 officials said, adding that 10 civilians were wounded in the clashes.
The U.S. military in northern Iraq confirmed that an attack on compound housing its Sunni allies against al Qaeda in Iraq near Sinjar killed five U.S.-allied fighters dead and five wounded. It said 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.
Mosul, 225 miles northwest of Baghdad, the provincial capital of Ninevah is believed to be the last major urban stronghold for al Qaeda in Iraq after many insurgents were driven north by U.S.-led offensives in Baghdad and surrounding areas.
Iraqi Prime Minister Nouri al-Maliki has promised a "decisive battle" against the terror network there 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 Qaeda front group for northern Iraq warned last week in an Internet statement that it was launching its own campaign in Mosul and surrounding areas and urged volunteers to join them to carry out suicide attacks on U.S. troops, Iraqi Shiites and the Kurdish peshmerga troops.
Meanwhile, two Iraqi Army officers were severely wounded in a drive-by shooting as they drove to work through western Baghdad, police said.
Gunmen opened fire on the officers' car in the predominantly Sunni Yarmouk neighborhood around 9 a.m., another officer said on condition of anonymity because he was not authorized to release the information.
Brig. Mohammed Bassem Abdul-Ridha and Col. Firqad Salman Alwan - who work at the Defense Ministry's General Inspector Office - were both severely injured, he said.
Sen. Ensign: We Shouldn't Leave Iraq Soon
Sen. John Ensign, R-Nev., rejected calls Saturday for the withdrawal of U.S. troops in Iraq by the end of the year, saying the decision should be based on "conditions on the ground and not on politics."
Speaking to reporters by conference call from Baghdad, Ensign said he now has "much more optimism" about the war in Iraq because of improved security following last year's increase in American troops.
His comments came amid a weekend tour of Iraq with Republican Sens. Tom Coburn of Oklahoma and Jim DeMint of South Carolina. The contingent met Saturday with top Iraqi leaders, as well as with Gen. David Petraeus, the top U.S. military commander in Iraq, and Ryan Crocker, the U.S. ambassador to Iraq.
The visit came two days after Rep. John Murtha, D-Pa., chairman of the House defense appropriations subcommittee, said he's preparing legislation that would give President Bush the war funding he wants this year, but on the condition that troops leave Iraq by the end of the year.
Ensign said "it's impossible to say" how long American troops will remain in Iraq, but Iraqi leaders warned the contingent that a pullout "too soon" would embolden Iran and al Qaeda.
"It should be based on conditions on the ground and not on politics," he said. "I strongly believe the American policy in Iraq should be to ensure a stable Iraq ... able to defend itself not only from within but from without. If we leave now, Iran will over-run it."
Petraeus has said he did not want to see the remainder of U.S. forces cut back too quickly after the withdrawal of an extra 30,000 troops by summer - a move that would leave roughly 130,000 to 135,000 troops, the same number as before President Bush sent the reinforcements.
Petraeus is scheduled to report to the president and Congress in April on possible additional cutbacks and any recommended changes in strategy.
Ensign, a strong supporter of the Bush administration's war policy, said he saw progress firsthand Saturday when he visited a town south of Baghdad.
"In November, it was a disaster and al Qaeda was completely in control," Ensign said. "Today it was a situation where I could walk down the street and interact with children and local merchants. I felt no danger whatsoever to my life."
Ensign said the vast majority of Iraq, including Baghdad, can report similar progress.
The senators' visit came a day after four American troops were killed in roadside bombings in Baghdad.
Ensign attributed the changes to greater cooperation from local sheiks, improved Iraqi security forces and more armed volunteers who help American troops patrol streets.
"Overall, I come away with much more optimism about what's happening in Iraq, although there are still major challenges ahead," he said. "While still significant, al Qaeda is in little pockets and separated from each other."
The senators were scheduled to spend the weekend in Iraq and visit Afghanistan on Monday before returning to Washington.
Ensign said he's also more optimistic because of political progress in Iraq that is "not widely reported in America."
"The Iraqi people are rejecting the insurgency and al Qaeda and the civil war that it looked like last year," he said. "They're tired of it and they want economic prosperity."
Al Qaeda Diaries And Memos
The military revealed two documents discovered by American troops in November: a 39-page memo written by a mid- to high-level al Qaeda official with knowledge of the group's operations in Iraq's western Anbar province, and a 16-page diary written by another group leader north of Baghdad.
In the Anbar document, the author describes an al Qaeda in crisis, with citizens growing weary of militants' presence and foreign fighters too eager to participate in suicide missions rather than continuing to fight, said Rear Adm. Gregory Smith, a U.S. military spokesman.
"We lost cities and afterward, villages ... We find ourselves in a wasteland desert," Smith quoted the document as saying.
The memo cites militants' increasing difficulty in moving around and transporting weapons and suicide belts because of better equipped Iraqi police and more watchful citizens, Smith said.
The author of the diary seized near Balad 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 switch by the Sunni tribes, whose resulting U.S.-backed groups are often referred to as awakening councils, has been credited with helping reduce violence across the country.
The councils were key to helping push al Qaeda out of Anbar province, once one of the country's most violent. The terror group's top leaders are now based somewhere in northern Iraq, Smith said, having moved out of Anbar and into Diyala province last year.
The U.S. military described both documents, but allowed reporters to see just four pages from them, citing security reasons.
The documents tell "narrow but compelling stories of the challenges al Qaeda in Iraq is facing," Smith told reporters in Baghdad's heavily guarded Green Zone.
"This does not signal the end of al Qaeda in Iraq, but it is a contemporary account of the challenges posed to terrorists from the people of Iraq," Smith said.
He said the documents are believed to be authentic, Smith said, because they contain details that only al Qaeda in Iraq leaders could know about battlefield movements and tactics.