A gas explosion in a coal mine in Iran's South Khorasan province killed at least 51 people and injured 20, Iran's state media said on Sunday.

The explosion occurred at 9pm local time on Saturday, state media said.

The accident was caused by a methane gas explosion in two blocks of the mine run by the Madanjoo company.

There were 69 workers in the blocks at the time of the explosion, state TV reported.

A rescue operation in one of the two blocks has been completed, finding 30 of the 47 workers who were in this block during the blast died, South Khorasan governor Ali Akbar Rahimi told state television on Sunday. Seventeen were injured.

Iran's Red Crescent said search and rescue operations were underway in the mine, where some workers remained trapped.

But "gas accumulation in the mine" has made the search operations difficult, local prosecutor Ali Nesaei was quoted by IRNA as saying.

"Currently, the priority is to provide aid to the injured and pull people from under the rubble," Mr Nesaei said.

"Seventeen injured people were transported to the hospital and 24 people are still missing," state media reported, citing the head of Iran's Red Crescent.

Another rescue operation in the second block is underway, but delayed by high methane density in the area. 

"Seventy-six per cent of the country's coal is provided from this region and around eight to 10 big companies are working in the region, including Madanjoo company," Mr Rahimi said. 

Iran President Masoud Pezeshkian offered his condolences to families of the blast victims and ordered a probe into the incident.

"Unfortunately, we learned that an accident occurred in one of the coal mines in Tabas and a number of our compatriots lost their lives. I offer my condolences to their respected families," Mr Pezeshkian said in remarks carried on state TV..

First Vice President Mohammad Reza Aref spoke with cabinet members to ensure "emergency follow-up" and support for the victims and their families, IRNA said.

Mr Nesaei said "the negligence and fault of the relevant agents will be dealt with" later on.

An explosion at a coal mine in the northern city of Damghan killed six people last year, also likely the result of a methane leak according to local media.

Two miners died in a collapse at the same site in May 2021, local media reported at the time.

Reuters/AFP