Mikhail Gorbachev, the last president of the Soviet Union, died on Tuesday after a prolonged illness, Reuters reported. Gorbachev was 91.

He was suffering from a kidney ailment, but the cause of his death is not yet known.

Gorbachev is credited for helping end the Cold War between the Soviet Union and the United States and its allies. However, he could not prevent the collapse of the Soviet Union – a group of East European countries led by Russia – in 1991.

Gorbachev had resigned from his post as president on December 25, 1991. He was awarded the Nobel Peace Prize in 1990 for the “leading role he played in the radical changes in East-West relations”.

On Wednesday, Russian President Vladimir Putin expressed grief over the death of Gorbachev.

“In the morning he will send a telegram of condolences to his family and friends,” the Kremlin spokesperson Dmitry Peskov said, according to the BBC.

United Nations Secretary-General António Guterres wrote on Twitter that Gorbachev was a “one-of-a-kind statesman who changed the course of history”.

“The world has lost a towering global leader, committed multilateralist, and tireless advocate for peace,” Guterres added. “I’m deeply saddened by his passing.”

British Prime Minister Boris Johnson said he admired Gorbachev’s courage and integrity.

“In a time of Putin’s aggression in Ukraine, his tireless commitment to opening up Soviet society remains an example to us all,” Johnson said.

Several other leaders also condoled the death of the former Soviet Union leader.