A company representative from the mine in the Prairie province of Saskatchewan told local television that rescuers were attempting to reach about 70 workers after a fire broke out on Sunday.
Marshall Hamilton, a spokesman for Mosaic, which operates the mine in Esterhazy, said the company was in radio contact with some of the miners.
"We have approximately 70 miners underground," he said. "We are in regular contact with approximately 40 of them. There are 30 of them that are in an area where the communication link we believe was destroyed by the fire. We believe they're safe. We've seen evidence that they've sealed themselves off."
CBC Television said miners at the operation reported smoke about a kilometre underground early on Sunday morning.
Hamilton said the miners were trained to seek out refuge stations, which are specifically built and designed for such incidents.
"In those refuge stations, the workers can seal themselves in and be safe with enough oxygen, food and water to be comfortable for 36 hours at the least," he said.
Hamilton said the fire was "the trickiest one that we've encountered in a quite a few years".
He said the families of those trapped had been contacted.