More than 600 pilgrims in Iraq were briefly hospitalised with respiratory problems after inhaling chlorine as a result of a leak at a water treatment station, authorities said on Sunday.

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

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

There, they will mark the Arbaeen — the 40-day period of mourning during which the community commemorate the death of Husain, grandson of Prophet Muhammad (PBUH).

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

“All have received the necessary care and left the 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.