Gorbachev died age 91 on Tuesday following a long illness, according to Russian state news agencies. He was the final leader of the Soviet Union, serving from 1985 until its collapse in 1991.

Russian President Vladimir Putin expressed his condolences and sent a message to Gorbachev's family and friends on Wednesday.

In an official letter published on the Kremlin's website, Putin expressed his "deepest condolences" to Gorbachev's relatives and friends, saying the former Soviet leader "had a huge impact on the course of world history."

Mr Gorbachev took power in 1985 and introduced reforms, as well as opening up the Soviet Union to the world.

But he was unable to prevent the slow collapse of the union, and many Russians blamed him for the years of turmoil that ensued.

Outside Russia, he was widely respected, with the UN chief saying he had "changed the course of history".

In recent years, his health had been in decline and he had been in and out of hospital. In June, international media reported that he had been admitted after suffering from a kidney ailment, though his cause of death has not been announced.

He will be buried in Moscow's Novodevichy cemetery, the resting place of many prominent Russians. It is not clear whether he will receive a state funeral.

President Vladimir Putin sent his "deepest condolences", describing how Mr. Gorbachev had had "a huge impact in the course of history".

"He deeply understood that reforms were necessary, he strove to offer his own solutions to urgent problems," the Russian leader said.

The two men had a strained relationship - their last meeting reportedly in 2006.

Most recently, Mr Gorbachev was said to have been unhappy with Mr Putin's decision to invade Ukraine, even though he had supported the annexation of Crimea in 2014.