Congratulations poured in from around the world, with President Vladimir Putin, US leader Joe Biden and German Chancellor Angela Merkel all sending their best wishes, he added.

"He is in quarantine in hospital for the duration of the pandemic," Vladimir Polyakov, spokesman for the Gorbachev Foundation, told AFP. 

"He is tired of this, like the rest of us."

In power between 1985 and 1991, Gorbachev pushed for reforms to achieve "glasnost" (openness) and "perestroika" (restructuring) but his policies eventually led to the demise of the Soviet Union.

After the Berlin Wall fell, he won the 1990 Nobel Peace Prize for "the radical changes in East-West relations."

Gorbachev, the first Russian leader to reach the age of 90, will mark his birthday with family and friends and has already received a "heap" of messages from around the world, his spokesman said.

Gorbachev would talk to his family and friends in a socially-distanced setting, possibly by video link. "We've set everything up," Polyakov added.

He said that Gorbachev had passed the time in isolation "editing books and articles".

Courage and integrity

In his message earlier Tuesday, Putin described Gorbachev as an "outstanding" politician.

"You rightly belong to a series of bright and outstanding people, distinguished statesmen of the modern age who have significantly influenced the course of domestic and world history."

He praised Gorbachev's "energy and creative potential", noting he remained involved in social and humanitarian projects.

The two Russian leaders past and present have had a complicated relationship.

Gorbachev has alternated between subtle criticism of the former KGB officer and praising him for bringing a level of stability to Russia.

Putin for his part has dismantled much of what the Soviet leader worked to achieve in guaranteeing liberties like free speech. 

He also famously referred to the Soviet collapse as "the greatest geo-political catastrophe of the century".

Merkel said the people of Germany would not forget Gorbachev's contribution to the country's reunification.  

"Today you can look back on your life's work with pride," she said.

UK Prime Minister Boris Johnson said he and the British people "remain in admiration of the courage and integrity you showed in bringing the Cold War to a peaceful conclusion."

While Gorby -- as he is affectionately known outside Russia -- is feted in the West, his reputation at home remains controversial. 

But even the Kremlin-friendly newspaper Moskovsky Komsomolets said Tuesday that Gorbachev had plenty to celebrate.

"He's the first leader in the country's thousand-year history who voluntarily resigned his post and remained alive and free," it said.

Government newspaper Rossiiskaya Gazeta suggested that the Soviet Union's demise was ultimately not his fault.

"Gorbachev came too late. It was very difficult to halt the destruction," it said.

"Gorbachev came too early. We were not ready then to appreciate and implement what was conceived," the newspaper added.