The "horrible accident" occurred between Dujail and Samarra, state news agency INA said, citing Khaled Burhan, director of health services in Salaheddin province.

The minibuses crashed into each other shortly before midnight Friday, leaving 18 people dead and 15 injured, a medical official in Salaheddin told AFP, speaking on condition of anonymity because he was not authorised to speak to the press.

One of the drivers was believed to have fallen asleep at the wheel, the same source said, citing the accounts of witnesses.

Among the dead were 14 Iranians, two Afghans and two people yet to be identified, another hospital official said, also speaking on condition of anonymity.

The dead included the two drivers, said an official from the area's traffic authority.

That official, who also referred to reports of a driver falling asleep, said one of the minibuses had veered into the opposite lane.

INA had initially reported a toll of 16 dead and 13 injured and said that those killed were Shiite Muslim pilgrims from neighbouring Iran.

Almost exactly a year ago, on September 11, 11 Iranian Shiite pilgrims and their Iraqi driver died when their minibus collided with a truck in Babil province, south of Baghdad, a health official said at the time.

Millions of Shiite pilgrims, many of them from Iran, head each year to the holy shrine city of Karbala for Arbaeen, one of the world's biggest religious gatherings.

Arbaeen marks the end of the 40-day mourning period for the killing of Imam Hussein -- a founding figure in Shiite Islam and grandson of the Prophet Mohammed -- by the forces of the caliph Yazid in 680 AD.

More than 2.6 million pilgrims have flown into Iraq or crossed its land borders since Arbaeen began this year, according to figures issued on Friday by Iraq's interior ministry.

Road accidents are a recurring accompaniment to Arbaeen, which concludes September 6-7 this year.

On Monday and Tuesday, four road accidents claimed the lives of 20 people and injured dozens more, mostly Iranian pilgrims. Those accidents occurred in the southern provinces of Wassit and Dhi Qar, near the border with Iran.

Conflict, neglect and endemic corruption have left oil-rich Iraq's infrastructure, including roads and bridges, in disrepair. Officials also say speed, mobile phone use and driving while impaired contribute to crashes.

Last year in Iraq, road accidents claimed the lives of more than 4,900 people, an average of 13 per day, according to health ministry data.