Moscow, Russia - Mikhail Gorbachev, Russian Nobel Peace laureate and the final leader of the Soviet Union, has died in Moscow at the age of 91.

"This evening, after a serious and long illness, Mikhail Sergeyevich Gorbachev died," the Central Clinical Hospital (CCH) in the Russian capital announced late on Tuesday.

The statesman is to be buried next to his wife in Moscow's Novodevichy Cemetery, the burial place of many of Russia's most famous politicians, writers, and composers.

The internationally respected statesman was often credited with bringing the Cold War to a peaceful end and for his instrumental role in German reunification.

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In the 1980s, under Gorbachev's leadership, the Soviet Union concluded groundbreaking treaties with the United States on nuclear disarmament and arms control, while at home, Gorbachev initiated an reform process with his policies of "glasnost" (openness) and "perestroika" (restructuring), which brought unprecedented freedoms to millions of people.

In 1990, Gorbachev received the Nobel Peace Prize for his courageous reforms. However, the massive economic upheavals that ensued across the Soviet Union ultimately led to the collapse of the 15-nation communist empire and to Gorbachev's own political downfall, when the country he led ceased to exist in 1991.

A large part of the Russian population always saw the former party and state leader as the gravedigger of the Soviet Union – and as a politician without an instinct for power – and Gorbachev was never to enjoy the massive popularity he had in the West in his homeland.

Gorbachev resigned as president of the Soviet Union in 1991 as the vast state voted to dissolve itself, creating 15 independent new countries. Staunch Gorbachev critic Boris Yeltsin became an independent Russia's first president in the same year, and Gorbachev was largely written off as yesterday's man.

In the three decades between his fall from power and his death, however, Gorbachev made significant contributions to Russian civil society with his eponymous foundation advocating democratic values and a Russian rapprochement with the West.

Gorbachev also wrote numerous books, including, most recently, one about his disappointment with Germany and the West. Specifically, he lamented what he saw as Russia constantly being cast as the enemy.

Unable to attend the 30th anniversary of the fall of the Berlin Wall in autumn 2019 for health reasons, Gorbachev received regular hospital treatment in recent years.

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Gorbachev was co-founder of the newspaper Novaya Gazeta, which is critical of the Kremlin and exposes abuses in Russia. In recent years, Gorbachev had repeatedly called on Russian President Vladimir Putin not to further restrict the freedom of the media and elections.

Putin expressed his sympathies upon hearing of Gorbachev's death and would be sending a telegram of condolence to the Gorbachev family on Wednesday morning, Kremlin spokesperson Dmitry Peskov said late on Tuesday evening.

It is not clear which international leaders might be able or willing to travel to Moscow for Gorbachev's funeral amid Russia's continuing war on neighboring Ukraine.

Nevertheless, tributes from world leaders began to pour in after news the former Soviet leader's death.

UN Secretary General António Guterres called Gorbachev a "one-of-a kind statesman who changed the course of history," in a statement expressing his condolences.

"He did more than any other individual to bring about the peaceful end of the Cold War," Guterres said.

Guterres noted that when Gorbachev received the Nobel Peace Prize in 1990, he observed that "peace is not unity in similarity but unity in diversity." This was an insight he put into practice by pursuing negotiation, reform, transparency, and disarmament, the UN chief said.

"The world has lost a towering global leader, committed multilateralist, and tireless advocate for peace," Guterres said.

US President Joe Biden also praised Gorbachev in a White House statement as a "man of remarkable vision" and one of only a few high-ranking Soviet officials had had the courage to admit that things needed to change.

"As leader of the USSR, he worked with President Reagan to reduce our two countries' nuclear arsenals, to the relief of people worldwide praying for an end to the nuclear arms race."

Even years after leaving office, Gorbachev was still deeply engaged, Biden added, recalling a 2009 visit the former Soviet leader made to the White House during which the two had spoken at length about how to reduce US and Russian nuclear stockpiles.

"It was easy to see why so many worldwide held him in such high esteem," Biden concluded.