Mikhail Gorbachev, who changed the course of history by triggering the demise of the Soviet Union and was one of the great figures of the 20th century, has died in Moscow aged 91.

His death was announced on Tuesday by Russian news agencies, which said Gorbachev had died at a central hospital in Moscow "after a serious and long illness".

Gorbachev, in power between 1985 and 1991, helped bring US-Soviet relations out of a deep freeze and was the last surviving Cold War leader.

His life was one of the most influential of his times, and his reforms as Soviet leader transformed his country and allowed Eastern Europe to free itself from Soviet rule.

The changes he set in motion saw him lionised in the West -- he won the Nobel Peace Prize in 1990 -- but also earned him the scorn of many Russians who lamented the end of their country's role as a global superpower.

He spent much of the past two decades on the political periphery, intermittently calling for the Kremlin and the White House to mend ties as tensions soared to Cold War levels after Russia annexed Crimea in 2014 and launched an offensive in Ukraine earlier this year.

- 'One-of-a-kind' -

His relationship with President Vladimir Putin was difficult at times, but the Russian leader nonetheless expressed his "deep sympathies" after Gorbachev's death.

"In the morning, (Putin) will send a telegram of condolences to his family and friends," Kremlin spokesman Dmitry Peskov told Russian news agencies.

Gorbachev spent the twilight years of his life in and out of hospital with increasingly fragile health and observed self-quarantine during the pandemic as a precaution against the coronavirus.

Gorbachev was regarded fondly in the West, where he was affectionately referred to as Gorby and best known for defusing US-Soviet nuclear tensions in the 1980s as well as bringing Eastern Europe out from behind the Iron Curtain.

He won the Nobel Peace Prize for negotiating a historic nuclear arms pact with US leader Ronald Reagan, and his decision to withhold the Soviet army when the Berlin Wall fell a year earlier was seen as key to preserving Cold War peace.

He was also championed in the West for spearheading reforms to achieve transparency and greater public discussion that hastened the breakup of the Soviet empire.

In a statement, US President Joe Biden credited Gorbachev with having "the imagination to see that a different future was possible and the courage to risk his entire career to achieve it".

"The result was a safer world and greater freedom for millions of people," he added.

British Prime Minister Boris Johnson, meanwhile, said he "always admired the courage and integrity" Gorbachev showed in bringing the Cold War to a peaceful conclusion.

"In a time of Putin's aggression in Ukraine, his tireless commitment to opening up Soviet society remains an example to us all," he said in a Twitter post.

UN chief Antonio Guterres praised Gorbachev as "a one-of-a-kind statesman who changed the course of history" and "did more than any other individual to bring about the peaceful end of the Cold War".

- 'Man of peace' -

French President Emmanuel Macron praised him as a "man of peace whose choices opened up a path of liberty for Russians. His commitment to peace in Europe changed our shared history."

The first Russian leader to live past the age of 90, he was congratulated by world leaders, including US President Joe Biden and former German chancellor Angela Merkel on his 90th birthday.

At home, Gorbachev remained a controversial figure and had a difficult relationship with Putin.

For Putin and many Russians, the breakup of the Soviet Union was a tragedy, bringing with it a decade of mass poverty and a weakening of Russia's stature on the global stage.

Many Russians still look back fondly on the Soviet period, and Putin leans on its achievements to buttress Russia's claim to greatness and his own prestige.

As the USSR collapsed, Gorbachev was superseded by the younger Boris Yeltsin, who became post-Soviet Russia's first president.

From then on, Gorbachev was relegated to the sidelines, devoting himself to educational and humanitarian projects.

He made a disastrous attempt to return to politics and ran for president in 1996 but received just 0.5 percent of the vote.

Over the years, he saw many of his major achievements rolled back by Putin.

- Supporter of free press -

An early supporter of Russia's leading independent newspaper Novaya Gazeta, founded in 1993, he donated part of his Nobel winnings to help it buy its first computers.

But the newspaper, like Russian independent media across the board, came under increasing pressure during Putin's two-decade reign.

Novaya Gazeta, whose chief editor Dmitry Muratov last year won the Nobel Peace Prize, suspended publication in late March after Moscow's military intervention in Ukraine.

Gorbachev himself made no public statements about Russia's military action in Ukraine, though his foundation called for "an early cessation (to) hostilities and immediate start of peace negotiations".

Uniquely among Soviet leaders, Gorbachev made no secret of his warm and supportive relationship with his wife Raisa, an elegant woman who often appeared in public with him and whose premature death from cancer was a devastating blow.

A source close to the Gorbachev family told news agency TASS that he would be buried next to Raisa at Moscow's Novodevichy cemetery, the resting place of many other famous Russian figures, including Yeltsin.

Gorbachev and first lady Raisa, a life 'hand-in-hand'

Uniquely among Soviet leaders, Mikhail Gorbachev made no secret of his warm and supportive relationship with his wife Raisa, an elegant woman who often appeared in public with him and whose premature death from cancer was a devastating blow.

Her confident air and sense of style made Raisa a celebrity, boosting Gorbachev's image in the West.

She defied stereotypes of dowdy, retiring Soviet leaders' wives and clearly demonstrated that her husband represented a new generation.

But to Gorbachev she was much more, a partner and confidante, who travelled with him as he brought about momentous changes to the Soviet Union.

"We walked through our whole life holding hands," Gorbachev said in a 2012 documentary. "She had something magnificent about her... she was like a princess."

The couple met as students at Moscow State University in the early 1950s. Raisa came from the small village of Rubtsovsk near the border with Mongolia to study philosophy, while Gorbachev was a law student from the southern Russian town of Stavropol.

In a 2012 memoir titled "Alone with Myself", Gorbachev said Raisa at first was not interested in a relationship.

"I felt I was losing my head. I wanted to see Raisa and be wherever she was," he wrote, but Raisa was getting over a painful breakup and told him she did not want to date him.

"I told her that I could not fulfil her request, that for me it would just be a catastrophe. That was my confession of love," he said.

- A sense of style -

They first kissed in a Moscow park, when they went swimming in a lake and a thunderstorm suddenly struck.

"I remember Raisa's face in a flash of lightning, her scared, questioning eyes. I hugged her and clumsily but passionately started kissing her."

After their marriage in 1953, they moved to Stavropol, where Gorbachev began a rapid rise in the Communist Party that made him the youngest member of the Politburo, at age 49, by 1979. Raisa worked as a lecturer in philosophy and sociology.

In 1985, Gorbachev was elected general secretary of the Communist Party, taking over the world's biggest state and second superpower, Raisa at his side.

"All my life, wherever I was, Raisa and I did not stop our dialogue. When I became general secretary and president, I would call Raisa two or three times a day or she would call me."

Gorbachev was sometimes portrayed as henpecked, with Raisa as the imperious power behind the throne.

A popular joke imagined the couple in bed together. "Misha," Raisa coos affectionately: "I bet you never imagined that one day you'd be sleeping with the wife of the Soviet president."

But he hotly denied in his memoirs that Raisa influenced his political decisions.

"Those constant stories that she took political decisions or put pressure on me are nonsense. She didn't even know how the Politburo worked."

Her varied wardrobe of fitted suits and fur coats were also controversial in a country where such stylish clothes were inaccessible to most women.

She revealed later though that she used to quietly sell her clothes after a few wearings at a second-hand store because Gorbachev's salary as general secretary did not stretch to many outfits.

- 'I hope we will meet again' -

Health problems began to appear during the 1991 failed coup against Gorbachev, when the couple were held for three days in isolation.

Raisa suffered a mini-stroke and was unable to speak for several days. Television images of her return to Moscow showed a shocked and diminished figure.

In 1999, after her husband had been replaced by a new generation of Russian politicians, Raisa was diagnosed with leukaemia. She was treated for several months in a German clinic but died that year, aged 67.

Gorbachev was haunted by the memories of her final days.

"I return again and again to the last days in Raisa's life and the tortures that she had to go through," he wrote in his memoirs. "What more could I have done, or not done, in my life to avoid this terrible fate?"

After she died, he said, "I had never felt so lonely in my life... I hope that we will meet again."

In 2006 Gorbachev set up a foundation named after Raisa that raises funds to help children with leukaemia and other forms of cancer.

A year later he opened a clinic in Saint Petersburg that specialises in treating children with leukaemia and bears Raisa's name.

The couple had one daughter, Irina, who worked as vice president of The Gorbachev Foundation.