At least eight prominent Russian businessmen have reportedly died by suicide or in as yet unexplained accidents since late January, with six of them associated with Russia's two largest energy companies.

Four of those six were linked to the Russian state-owned energy giant Gazprom or one of its subsidiaries, while the other two were associated with Lukoil, Russia's largest privately owned oil and gas company.

Earlier this year, the company took the unusual public stance of speaking out against Russia's war in Ukraine, calling for sympathy for the victims, and for the end of the conflict.

Lukoil's chairman Ravil Maganov died this week after falling out of the window of a hospital in Moscow, according to Russian state news agency TASS.

Lukoil confirmed the death on Thursday in a statement published on its website.

Maganov "passed away following a severe illness," Lukoil said, making no mention of a fall. "Maganov immensely contributed to the development of not only the company, but of the entire Russian oil and gas sector."

Another top Lukoil manager, Alexander Subbotin, was found dead near Moscow in May after reportedly visiting a shaman, TASS reported. Russia's State News Agency quoted an official as saying authorities were called to an unconscious man suffering from a heart failure. TASS reported the police opened a criminal investigation into the case.