"Smoke appeared during the construction of a well. Nine workers were in the zone, and there is reason to believe that these people have died," said a statement issued by an enquiry commission in the Perm region, about 1,200 kilometres (750 miles) east of Moscow.
The commission said that eight other people who were also underground when smoke appeared were evacuated without injury, while Russian television showed smoke billowing from the mine.
The Russian group Uralkali, a leading potash producer, operates the mine but said in a statement that the nine missing workers were employed by a subcontractor.
Potash is a common ingredient in fertiliser.
The last major Russian mining accident occurred in August 2017, when eight workers disappeared during a flood in a Siberian diamond mine.
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