TEHRAN - There was little hope of finding any survivors at a mine in northern Iran where 21 miners died trying to rescue their trapped workmates, a rescue official said Thursday.

The 21 perished in a failed rescue bid after an explosion Wednesday caused a mile-long tunnel to partially collapse, news agencies said.

"The hope of finding the remaining 14 miners still alive is now minimal," Sadegh-Ali Moghadam, emergencies director of Golestan province, was quoted as saying by the official IRNA news agency.

Officials had earlier said 32 miners were feared to be trapped deep underground in the Zemestan Yort mine.

"The bodies of the 21 miners were found at a depth of 600 metres (yards)," Moghadam said, adding that operations were continuing to reach the workers stranded at a depth of 1,000 to 1,500 metres.

The tunnel caved in when methane gas exploded underground as workers tried to jump-start a locomotive engine, according to initial reports.

The rescue operation had been hampered as the tunnels were filled with poisonous gases and the miners were stuck at the far end of a 1,000-metre shaft in a mine that is around two kilometres (1.2 miles) long.

Three days of mourning were declared in the province of Golestan, where the accident occurred.

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Images broadcast on state television IRIB showed the evacuation of injured and exhausted miners on stretchers and relentless activity of the rescue services at the entrance of one of the mine tunnels.

Some 30 miners were treated in various hospitals for gas poisoning and injuries, state news agency IRNA reported.

There was no information on the condition of those trapped.

The workers sent to hospital were among those who had rushed from outside to help the victims after the blast.

Iran's President Hassan Rouhani has sent Minister of Labour and Welfare Ali Rabii to the mine in order to oversee the rescue operations and treatment of victims, IRNA reported.

First Vice President Eshaq Jahangiri expressed condolences to the families of the victims and tasked Minister of Mining and Industry Mohammad Reza Nematzadeh with investigating the cause of the accident.

The issue is particularly sensitive two weeks ahead of a presidential vote when Rouhani hopes to win re-election.

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

The mine has 500 workers and the explosion happened during a change of shift, state media said.-AFP