Sunni militants summarily executed 14 Shiites on Thursday after setting up a roadblock north of Baghdad, stopping trucks and checking the IDs of drivers to determine their religion, Iraqi officials said.
Attacks elsewhere killed another 26 people, bringing the death toll for July to more than 700 -- the deadliest month so far in a year marked by spiralling violence.
The night-time highway killings were reminiscent of the darkest days of the Sunni-Shiite sectarian bloodshed in Iraq in 2006-2007, when thousands of people were killed because of their religious affiliation or forced to abandon their homes under threat of death.
Lingering tensions between Sunnis and Shiites have been inflamed by persistent violence in Iraq and the civil war in neighbouring Syria, and there are growing fears that the country is slipping back towards all-out sectarian conflict.
Two local officials said some 150 militants carried out a coordinated operation during the night that included the highway killings, in the area of Sulaiman Bek, a town north of Baghdad.
The militants began by attacking the town itself with mortar rounds, rocket-propelled grenades and automatic weapons late on Wednesday.
That attack drew security forces away from the highway connecting Baghdad with the north, after which a group of around 40 militants broke off and set up the checkpoint.
They only maintained it for about half an hour but were able to stop dozens of truck drivers, and executed 14 who were Shiites.
"These criminals belong to what is called the Islamic State of Iraq, and they targeted Shiite drivers and left the Sunnis," local official Shalal Abdul Baban told AFP, referring to an Al-Qaeda front group.
"It was killing by ID," he said.
Iraqi identification cards list a person's name and place of birth, from which religious affiliation can be surmised.
The entire operation, including the attack on the town in which at least one person was wounded, lasted for about three hours, after which the militants withdrew.
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Iraqi soldiers surrounded Sulaiman Bek on Thursday and arrested about 150 people following the violence, local official Talib Mohammed al-Bayati said.
Sulaiman Bek was briefly seized by militants in late April, but the assailants later withdrew under a deal worked out by tribal leaders and government officials, allowing security forces to move back in.
The town's seizure came amid a surge of violence that began on April 23 when security forces moved in against anti-government protesters near the northern town of Hawijah, sparking clashes in which 53 people were killed.
Dozens more died in a wave of subsequent unrest including revenge attacks on the security forces.
The highway killings come just days after a highly coordinated assault by militants on two Iraqi prisons that saw hundreds of inmates escape, an operation claimed by an Al-Qaeda front group.
The militants attacked prisons in Abu Ghraib, west of Baghdad, and Taji, north of the capital, with mortar rounds, bombs and gunfire beginning on Sunday night, sparking clashes with security forces that lasted for 10 hours.
At least 500 prisoners, including senior Al-Qaeda members, escaped, and at least 20 members of the security forces and 21 prisoners died.
The latest attacks are indicative of the militants' growing reach in Iraq, and of the rapidly deteriorating security situation.
Militants carried out bombings and shootings in various other areas of the country on Thursday, killing a further 26 people, including three police and a soldier.
In the deadliest attack, a roadside bomb hit the main street in Nawfel, a town north of Baghdad, while another exploded when people gathered at the scene, killing a total of 15 people and wounding 15 more.
In Madain, south of Baghdad, a bomb in a cafe killed at least two people and wounded at least 10 -- the latest in a string of attacks to strike crowded nightspots in recent weeks.
And a civilian was shot dead during rare clashes between security forces and gunmen in Arbil, the capital of Iraq's three-province autonomous Kurdish region, which is usually spared the violence plaguing other regions.
Iraq has faced years of attacks by militants, but analysts say widespread discontent among members of its Sunni Arab minority that the government has failed to address has fuelled the surge this year.