The blast occurred at midday at a depth of 1,200 meters in the coal mine, located in Khoshyeylaq area in the northeastern province of Golestan.
Minister of Cooperative, Labor and Welfare Ali Rabiee, who has traveled to the disaster site, confirmed in the early hours of Thursday that 35 miners have been pronounced dead.
21 bodies have been retrieved from the mine, a local official said earlier.
Officials say the explosion occurred after a mine locomotive stalled at a depth of 700 meters and the workers tried to jump-start the vehicle with an external battery. Electric sparks triggered the explosion in the tunnel filled with methane.
Shahin Fathi, an official with Iran’s Red Crescent Society said Wednesday more than 40 miners had been trapped in the mine after the blast, six of whom were rescued with injuries.
The rescue operation is hampered by toxic gas in the tunnels. Miners were stuck at the far end of a 1,000-meter shaft in the mine that is around two kilometers long.
Efforts were underway to dig a side tunnel to access the stranded workers.
Three days of public mourning has been declared in Golestan Province, which lies along the Caspian Sea and borders Turkmenistan.
Khoshyeylaq is 375 kilometers northeast of Tehran.