Hopes of finding nine missing miners alive were fading more than 36 hours after the blast Wednesday at the mine in Azadshahr in Golestan province, where three days of mourning were declared.
The rescue operation at the Zemestan Yort mine was hampered by poisonous gases that filled the tunnels as well as by fears of a further collapse.
The removal of rubble blocking the tunnel was expected to finish by the end of Thursday, state television broadcaster quoted Labour Minister Ali Rabii as saying from the site.
"Efforts by various organisations will be rewarded (by God). They (should) increase their efforts as much as possible," he said in a message.
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President Hassan Rouhani also expressed sorrow for victims of the disaster, which has overshadowed his re- election campaign.
Tasnim news agency quoted director of Iran's emergency services Pir Hossein Kolivand as saying that 26 bodies had been recovered from the mine.
Sadegh-Ali Moghadam, emergencies director in Golestan province, had said earlier that 21 victims were found at a depth of 600 metres (yards) and rescue teams had descended 1,400 metres into the tunnel where the blast happened.
"The problem is the upper arch which has been weakened by the explosion. Miners are working to secure the tunnel" to be able to move forward, he said.
The tunnel caved in when methane gas exploded underground as workers tried to jump-start a locomotive engine, according to initial reports.
Rouhani ordered all departments "to use all equipment available to facilitate the rescue services and save those trapped".
"The painful incident involving a number of hard-working miners in the Azadshahr mine brought sorrow and mourning to all Iranian people," he said.
The army joined forces today with rescue teams by sending medics, bulldozers and trucks.
More than 70 miners were treated in hospitals for gas poisoning and injuries, state news agency IRNA reported.
The workers sent to hospital were among those who had rushed from outside to help the victims after the blast.
Rouhani sent Rabii along with mining and industry minister Mohammad Reza Nematzadeh to the mine to oversee the rescue operations, treatment of victims and lead an investigation.
"The government will undertake the payment of all workers' wages until the mine reopens," he added.
The mine has 500 workers and the explosion happened during a change of shift, state media said.