An explosion at a coal mine in northern Iran has left at least 22 dead and many more injured.

Unconfirmed reports suggested at least 35 had been killed in the incident at the Zemestanyurt mine on Wednesday, which lies nine miles from Azad shahr in the Golestan province.

Several officials blamed the explosion on accumulated gas and said it was affecting rescue efforts.

President Hassan Rouhani has issued an order demanding his government use all available resources to find those still trapped, according to the state-run IRNA news agency.

The province will observe three days of mourning, Iranian state television reported.

Provincial spokesman Ali Yazerloo said the blast happened at 12:45 p.m. local time (0945 GMT) on Wednesday.

There was confusion about how many miners had been trapped inside, with numbers ranging from dozens to up to 80.

At least 25 people were taken to hospital after inhaling gas during the search for survivors.

Golestan lies along Iran's northern border with Turkmenistan and along the shore of the Caspian Sea.

This is not the first disaster to strike Iran's mining industry. In 2013, 11 workers were killed in two separate mining incidents. In 2009, 20 workers were killed in several incidents. Poor safety standards and inadequate emergency services in mining areas often are blamed for the fatalities.