Norway's Yara ASA, which is among the world's largest fertiliser companies and a leading buyer of potash from Belaruskali, said the situation at the Belarusian firm was "not tenable".

Belaruskali did not immediately respond to a request for comment.

"We need to see imminent and  substantial improvements  in  the  conditions for, and relations with  Belaruskali's  workforce, and an end to the use of reprisals as well as improved occupational safety conditions," Yara Chief Executive Svein Tore Holsether said in a statement.

"We are particularly concerned by numerous reports of dismissals of workers who have expressed their democratic rights in a peaceful manner," he said.

Holsether, who visited Belaruskali in September to make his position clear, did not say whether Yara would sever business ties with the potash miner.

Belarus has been rocked by mass protests since an Aug. 9 presidential election which veteran incumbent Alexander Lukashenko said he won - an assertion contested by his opponents, who say the vote was rigged and want him to quit.

In August, a strike leader at Belaruskali was sentenced to 15 days in prison, and last month the company said 49 workers had been fired for taking part in a strike.

"Despite our continued  efforts  to bring about change through dialogue,  we have not seen significant improvements  in the situation for  Belaruskali  workers," Holsether said.

(Reporting by Terje Solsvik in Oslo; Additional reporting by Vladimir Soldatkin in Moscow; Editing by Helen Popper)