MOSCOW (AP) -- The investment vehicle of Russian billionaire Mikhail Prokhorov says it will buy a stake in troubled potash producer Uralkali.

Onexim said in a statement on Monday it has agreed to buy 21.75% in Uralkali from fellow billionaire Suleyman Kerimov. It did not disclose how much it would be paying, but its market price was around $3.5 billion in late Monday trading.

Russia's largest potash producer has been embroiled in a bitter row with its partners in Belarus. The company's chief executive was arrested in Belarus in August and has been kept under house arrest since. Belarusian authorities suspect him of "abusing his power" and are investigating Kerimov.

The case has been viewed as retaliation for Uralkali's decision to pull out of a venture with its Belarusian partner.