Tehran (AFP) - Rescuers battled hazardous conditions Thursday at a coal mine in northern Iran where 26 miners died as attempts failed to save workmates trapped deep underground after an explosion.

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.

Iran's supreme leader Ayatollah Ali Khamenei offered condolences for the "very bitter" tragedy.

"Efforts by various organisations will be rewarded (by God). They (should) increase their efforts as much as possible," he said in a message.

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 hope of finding the remaining miners alive "is now minimal," he told state IRNA news agency.

"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.

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- Gas poisoning -

Images broadcast on state television IRIB showed the evacuation of injured and exhausted miners on stretchers as rescue services worked relentlessly at the entrance to one of the mine tunnels.

The army joined forces on Thursday 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.

Rabii told IRNA that all work at the mine will be completely halted for six months until the probe is completed.

"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.

The issue is particularly sensitive two weeks before a presidential vote in which Rouhani hopes to win re-election.

Conservative candidates have accused the government of failing to care for the poor.