Iranian news agencies are quoting provincial officials as saying that 21 miners died in a coal mine explosion earlier in the day in the country's north.
The explosion hit a coal mine in the province of Golestan on Wednesday, trapping dozens of miners.
After nightfall, Iran's official IRNA news agency said bodies of 21 miners had been found.
The report quoted Sadegh Ali Moghadam, the provincial director general of disaster management, as warning of the "possibility of the death toll rising."
Moghadam said about 35 people are now thought to have been trapped inside the mine.
The semi-official ISNA and Tasnim news agencies gave the same death toll. ISNA quoted Houman Hadian of the Golestan mining industry, as saying the main tunnel of the mine is now open and that rescue teams are continuing their search.
Iranian news reports said the explosion happened while workers were changing shifts.
"I carried two out of the mine," an unidentified, soot-covered miner told state television. "It is not possible to go inside again. Oxygen tanks should be brought."
Another miner said he feared his colleagues trapped inside may have died.
"The gas in the mine exploded and my colleagues remained in the tunnel," he said.
Semi-official Iranian news agencies posted images online from the scene, showing ambulances and emergency workers gathered at the mouth of the mine. Some showed dazed workers, covered in coal dust, being helped by bystanders or laying on the ground as rescuers rushed past with oxygen bottles.
More than 500 workers are employed at the Zemestanyurt mine, which lies 14 kilometers (9 miles) from Azadshahr, according to IRNA. Golestan sits along Iran's northern border with Turkmenistan and along the shore of the Caspian Sea.
Oil-producing Iran is also rich in a variety of minerals. Iran annually consumes some 2.5 ton of coal but only extracts about 1 million tons from its mines per year. The rest is imported, often consumed in the country's steel mills.
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. Lax safety standards and inadequate emergency services in mining areas often are blamed for the fatalities.
Since Iran's nuclear deal with world powers, the country has begun an effort to renovate some of its coal mines. Delegations have visited Tehran from foreign countries including the Czech Republic, hopeful for contracts.