Former President of the Soviet Union Mikhail Gorbachev has died aged 91, Russian media reported on Tuesday.

"Mikhail Sergeevich Gorbachev died this evening after a serious and long illness," the Central Clinical Hospital was quoted as saying.

Russian President Vladimir Putin has expressed deep condolences over the death of Gorbachev, according to Russia's RIA Novosti news agency.

In a statement, United Nations Secretary-General Antonio Guterres described Gorbachev as "a unique leader who changed the world for the better."

World leaders including European Commission President Ursula von der Leyen and French President Emmanuel Macron also sent their condolences.

Born in March 1931, Gorbachev is viewed as one of the most significant figures in the second half of the 20th century. Having assumed the role of general secretary of the Communist Party of the Soviet Union in 1985, he pushed for drastic reforms that opened up the Soviet economy and won him adulation in the West.

While having played a significant role in ending the Cold War, his attempts to simultaneously overhaul the political system were deemed a key factor in the Soviet dissolution, which took place as he resigned as president in December 1991.

After his resignation, Gorbachev was mostly engaged in social and cultural activities.

(Cover: In this file photo taken on November 25, 2011, former Soviet Union President Mikhail Gorbachev gives a press conference in Montpellier, southern France. /CFP)