Sadegh-Ali Moqaddam, head of the emergency department of Golestan Province, said that up to 50 people are believed to be trapped in the mine.
Several officials blamed the explosion on accumulated gas which is now affecting even the rescue teams.
The reports said more than a dozen injured people had been transferred to hospital after the blast at 12:45 p.m. local time in the Zemestanyurt coal mine in Golestan province.
Forty miners were trapped in one section of the mine, while 30 to 40 others were trapped in another section, said Pir-Hossein Kolivand, head of Iran's Emergency Center.
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After nightfall, Iran's official IRNA news agency said the bodies of 21 miners had been found.
Press TV, the English language arm of Iranian state television, reported the deaths of the two miners.
Poisonous gas inside the tunnel has hampered the rescue work. Last year, the country produced 1.7 million tonnes of coal, an increase from previous years, thanks to the reduction of global sanctions. In 2013, 11 workers were killed in two separate mining incidents.
Since Iran's nuclear deal with world powers, the country has begun an effort to renovate some of its coal mines.