President Hassan Rouhani has traveled to the northeastern Iran province of Golestan where an explosion at a coal mine left at least 35 people dead.

Accumulation of methane gas at the local coal mine in the province’s Azad-Shahr triggered the blast on Wednesday, which also left 73 other workers injured. Twenty-two bodies have been retrieved belonging to those trapped.

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Rouhani flew to Golestan on Sunday, and immediately headed to Azad-Shar to address the situation and meet with the tragedy’s survivors and miners.

The president called for the explosion to be investigated "so that such disasters do not repeat themselves in the future."

Miners are seen outside Zemestan Yurt coal mine in Golestan province, May 7, 2017. (Photo by IRNA)

Authorities declared three days of mourning in the province, beginning on Thursday.

Ali Rabi’ei, the Iranian minister for cooperatives, labor, and social welfare, also traveled to Azad-Shahr, where he said many of the country’s 20-, 30-, and 50-year-old mines, especially coal mines, were in need of renovation and had to have their equipment replaced.

Miners are seen outside Zemestan Yurt coal mine in Golestan province, May 7, 2017. (Photo by IRNA)

'Some workers may still be alive'

He said some of those trapped might still be alive as they could have been working in a part of the mine, into which methane gas did not percolate at the time of the blast.

The debris from the blast equals that of a four-story building, he said, adding operations to remove the rubble might take five or six days or even longer.

This May 7, 2017 photo by IRNA shows rescue teams clearing the coal mine where an explosion killed 35 people.

Rescue workers are currently trying to reach the site of the explosion from a different direction, Rabi'ei said.

The mine, operated by the private sector, started 13 years ago, amid reports that the equipment used in the pit was in dire need of repair or replacement.

Miners are seen outside Zemestan Yurt coal mine in Golestan province, May 7, 2017. (Photo by IRNA)

“I had already received some reports about this mine concerning the contractor. Today, some new problems came to my attention," the minister said, without explanation.