At least 28 Pakistani pilgrims are killed in a bus accident in Iran.

A bus carrying Pakistani pilgrims overturned in Iran, killing at least 28 passengers, Iranian state media reported.

The accident occurred on Tuesday evening in the central Iranian province of Yazd and was caused by a defective braking system, Reuters reported, citing preliminary police investigations.

Another 23 passengers were injured, seven of them seriously, the province's crisis management director told state television.

The pilgrims traveled from Pakistan's Sindh province to the holy city of Karbala in Iraq to commemorate one of the greatest events in the Shiite calendar.

According to the Pakistani newspaper Dawn News, there were probably 53 people on the bus at the time of the accident, including pilgrims from Larkana, Ghotki and cities in Sindh.

Among those killed in the crash were 11 women and 17 men, said crisis management director Ali Malekzadeh.

According to local media reports, the bus caught fire in front of the Dehshir-Taft checkpoint in Iran, about 681 kilometers south of the capital Tehran.

Mr Malekzadeh said the bus went off the road “because the driver did not have control of the vehicle, was unfamiliar with the road, was speeding too high and there were technical problems”.

Pakistan's President Asif Ali Zardari said he would mobilize the Foreign Ministry to repatriate the bodies of those killed in the plane crash and provide assistance to the injured.

Meanwhile, Interior Minister Moshin Naqvi said: “We are deeply saddened by the loss of precious human lives in the bus accident in Iran.”

The Pakistani consulate in Iran has been asked to assist in the recovery efforts and Pakistan's ambassador to Tehran, Muhammad Mudassir Tipu, said he was in contact with the Iranian government and local authorities in Yazd.

The pilgrimage, known as Arbaeen (Arabic for “40”), marks the end of a 40-day mourning period for Imam Hussein, the grandson of the Prophet Muhammad.

Iman Hussein died in the Battle of Karbala over 1,000 years ago and this event still evokes feelings of deep grief and sadness among many believers.

Pilgrims from many parts of the world go to his tomb, which is hidden under a golden dome.

The ancient city of Karbala comes to life during this time and is decorated with fairy lights, flowers and pennants.

The Iraqis of Karbala are proud to host Arba'in pilgrims – reflecting a tradition of hospitality.

Last year's pilgrimage was attended by around 25 million people, making Arbaeen the largest annual pilgrimage in the world.

In contrast, in the same year, around 2.5 million pilgrims took part in the Hajj to Mecca.