Between 21 and 35 workers were killed and scores were injured after a huge explosion occurred at the Zemestanyurt coal mine in northeastern Iran on May 3, state media reported on May 4.

Various Iranian news reports said that the blast happened during a change of shift around 12:45 local time when workers tried to jump-start a locomotive at the mine, located 14km north of Azadshahr in Golestan province.

Ali Rabiei, minister of cooperatives, labour and social welfare was quoted as providing the death toll by semi-official Tasnim news agency. He was sent to the mine by President Hassan Rouhani to oversee the rescue operations.

Some 25 people who entered the mine to try to help the trapped miners were taken to hospital due to gas inhalation, according to Press TV, the English service of Iranian state television.

As rescue teams continued to work frantically through the night to reach trapped victims, Sadegh Ali Moghadam, the provincial director of disaster management, was quoted by Iran's official news agency IRNA as warning that the death toll may rise.

"We are trying to inject oxygen into the tunnel to increase the chances of survival for people who are trapped in the mine," Moghadam said, according to an AP account of local news reports.

Although Iran has the second largest proven gas reserves in the world, many rural areas in the country of 80 million continue to use coal for heating during the cold months. The mine is located in one of two areas in Iran that boast heavy coal deposits, according to the Iranian Mines and Mining Industries Development and Renovation Organisation.The other area is South Khorasan in eastern Iran. 

More than 500 workers are employed at the mine, believed to be one of the biggest in the region.

Iran’s extracted some 1.68mn tonnes of coal and produced 602,958 tonnes of coal concentrate in the last Persian calendar year (ended March 20).