Dozens of Sunni militants set up a roadblock north of Baghdad early Thursday, stopped trucks, checked the IDs of drivers and summarily executed 14 Shiites, Iraqi officials said.
The attack was reminiscent of the darkest days of the Sunni-Shiite sectarian bloodshed in Iraq in 2006-07, when thousands of people were killed because of their religious affiliation or were 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.
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 seizure of the town 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 against 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.
INTERPOL said in an online statement on Wednesday that the escapes "constituted a major threat to global security", and that it had issued a regional security alert at Iraq's request.
The latest attacks are indicative of the militants' growing reach in Iraq, and of the rapidly deteriorating security situation.
Another seven people were killed elsewhere on Thursday, including three policemen.
Also killed was a civilian 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 areas.
More than 680 people have now been killed in unrest in July, making it the deadliest month in a year marked by spiralling violence.
Iraq has faced years of attacks by militants, but analysts say widespread discontent among members of its Sunni Arab minority, which the government has failed to address, has fuelled the surge this year.