At least 21 people have been killed and dozens injured after an explosion in a coal mine in northern Iran.

The blast happened in the Zemestanyurt mine when workers tried to jump-start a locomotive at around 12.45pm local time (8.45am BST) on Wednesday during a change of shift.

The semi-official Fars news agency reports that rescue efforts were hampered when more than 50 workers were trapped in a 2km-long tunnel filled with gas.

A regional official told Tasnim news agency: "21 bodies of workers have been taken out of the mine ... so far, there have been 69 injured."

More than 30 people have been taken to nearby hospitals for treatment.

Around 500 people work at the mine, whose rescue operation is being overseen by the country's minister of labour and welfare.

Iran is a major coal-producing country, producing 1.7m tons last year, Reuters reports. It uses it primarily for domestic steel production.