The incident took place overnight on the route between the two Shi’ite holy cities of Najaf and Karbala, located in the centre and south of Iraq respectively.

This year, several million Shi’ite Muslim pilgrims are expected to make their way to Karbala, which houses the shrines of the revered Imam Hussein and his brother Abbas.

There, they will mark the Arbaeen – the 40-day period of mourning during which Shi’ites commemorate the death of Hussein, grandson of the Prophet Muhammad.

In a brief statement, Iraq’s health ministry said “621 cases of asphyxia have been recorded following a chlorine gas leak in Karbala”.

“All have received the necessary care and left hospital in good health,” it said.

Security forces charged with protecting pilgrims meanwhile said the incident had been caused by “a chlorine leak from a water station on the Karbala-Najaf road”.

Much of Iraq’s infrastructure is in disrepair due to decades of conflict and corruption, with adherence to safety standards often lax.

In July, a massive fire at a shopping mall in the eastern city of Kut killed more than 60 people, many of whom suffocated in the toilets, according to authorities.