The circumstances of Lukoil chairman Ravil Maganov's death were unclear.

The state news agency Tass cited an unnamed law enforcement source as saying Maganov committed suicide while being treated at Central Clinical Hospital in Moscow. The report said he had been admitted there for a heart attack and was taking antidepressants.

A Lukoil statement Thursday said Maganov ``passed away after a severe illness'' but did not give details.

Lukoil was one of the few Russian companies to publicly censure Russia's military operation in Ukraine, calling in March for the ``immediate cessation of the armed conflict.''

Maganov is the latest in a series of Russian businessmen, particularly in the energy industry, who have died suddenly in unclear circumstances this year.

His body was found on the grounds of the hospital, where Russia's political and business elite are often treated, the reports said.