He said all the bus passengers hailed from Pakistan. 

Fifty-one people were on board at the time of the crash outside of the city of Taft, 500km southeast of the Iranian capital, Tehran. 

Iranian state television later broadcast images of the bus, turned upside down on the highway with its roof smashed in and all its doors open. 

Rescuers stepped gingerly through the broken glass and debris littering the road. 

In the state TV report, Malekzadeh blamed the crash on the bus brakes failing and a lack of attention by its driver. 

In Pakistan, media reports quoted a local Shi'ite leader, Qamar Abbas, saying as many as 35 people had died in the crash. 

He described those on the bus as coming from the city of Larkana in Pakistan's southern Sindh province. 

Pakistan's Interior Minister Mohsin Naqvi expressed his condolences over the deaths. 

“We are deeply saddened by the loss of precious human lives in the bus accident in Iran,” he said.

Iran has one of the world’s worst traffic safety records with some 17,000 deaths annually.

The grave toll is blamed on wide disregard for traffic laws, unsafe vehicles and inadequate emergency services in its vast rural areas.

The pilgrims had been on their way to Iraq to commemorate Arbaeen, which marks the death of the Prophet Muhammad’s grandson, Hussein, at the hands of the Muslim Umayyad forces in the Battle of Karbala, during the tumultuous first century of Islam’s history. 

Hussein was seen by his followers as the rightful heir of the prophet’s legacy. 

When he refused to pledge allegiance to the Umayyad caliphate, he was killed in the battle, cementing the schism between Sunni and Shi'ite Islam. 

Pilgrims gather in Karbala, Iraq, in what’s regarded as the largest annual public gathering in the world, drawing tens of millions of people each year. 

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