Russian billionaire Mikhail Fridman, whose private equity firm LetterOne controls a one-third share of German oil and gas company Wintershall Dea, has spoken out publicly against Moscow's invasion of Ukraine, calling for an end to the bloodshed.

In a letter to LetterOne employees published on the company's website, Fridman, who was born in Ukraine and whose parents still live in the western city of Lviv, expressed his "deep concerns".

"I have also spent much of my life as a citizen of Russia, building and growing businesses.