Hopes of finding 14 missing miners alive were fading 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. 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.