The vehicle carrying the pilgrims slammed into the parked truck in Babil province south of Baghdad, provincial health authorities spokesman Ahmed al-Jibouri told AFP.

"Eleven Iranians died on impact, along with their Iraqi driver," he said, adding that four other pilgrims were in critical condition.

He said a fire had erupted, and that the driver of the truck had been taken in for questioning.

Iran's deputy interior minister Majid Mirahmadi said that "due to the seriousness of the accident and the fire, the bodies cannot be identified".

Those of the Iranian victims "will be transferred to Iran for identification", he told Iran's Mehr news agency.

Conflict, neglect and endemic corruption have left oil-rich Iraq's infrastructure, including roads and bridges, in disrepair.

Many roads are full of potholes and are plunged into total darkness at night.

But officials also say speed, mobile phone use and driving under the influence contribute to crashes.

In July, the health ministry said 4,800 people died in traffic accidents last year in Iraq, or more than 13 a day.

According to official statistics, almost two million Iranian pilgrims have entered Iraq to attend this year's Arbaeen pilgrimage -- one of the world's biggest religious gatherings -- in the Shiite shrine city of Karbala.

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.