Red Crescent ambulances at the scene of a blast at a coal mine in Tabas in Iran's Khorasan province on Sunday. AFP

"The number of dead workers increased to 51" in the explosion at the Tabas Mine in eastern Iran, the official IRNA news agency reported, revising an earlier death toll of 30. It added that 20 other people were injured.

The explosion occurred at around 9:00pm on Saturday, when around 70 workers were present at the site in South Khorasan province, IRNA said.

According to the report, a leak of methane gas led to the blast in two blocks of the mine, which is owned by private Iranian firm Madanjoo.

State TV broadcast footage of ambulances and helicopters arriving at Tabas to transport the injured to hospital.

Online footage carried by IRNA showed bodies of some of the victims, wearing their work uniform, carried out of the site on mining carts.

South Khorasan governor Javad Ghenaat told state TV that rescue teams were working to recover the remaining bodies. An apparent "gas condensation" in the mine is believed to be the cause of the explosion, said Iranian Interior Minister Eskandar Momeni, who was headed to Tabas.

Iran's President Masoud Pezeshkian, in remarks carried state TV before departing for the UN General Assembly in New York, offered his condolences to the families of the victims and ordered a probe into the deadly 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," Pezeshkian said.

His 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.

The authorities have announced three days of public mourning in the eastern province after the deadly blast, according to local media.

Trapped

Iran's Red Crescent said search and rescue operations were underway in the mine, where some workers remained trapped. According to IRNA, they were about 250 metres below the surface, cut off from rescuers by chambers that had filled up with concentrated methane gas. "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," Nesaei said. He added that "the negligence and fault of the relevant agents will be dealt with" later on.

The Tabas mine covers an area of more than 30,000 square kilometres (nearly 11,600 square miles) and holds mass reserves of coking and thermal coal, according to IRNA. It is "considered the richest and largest coal area in Iran," IRNA said.

Agence France-Presse