MOSCOW (AP) — George Blake, a former British intelligence officer who worked as a double agent for the Soviet Union, has died in Russia. He was 98.

Russia's Foreign Intelligence Service, known as SVR, announced his death Saturday in a statement, which didn't give any circumstances of his death. Russian President Vladimir Putin expressed condolences, hailing Blake as a “brilliant professional” and a man of “remarkable courage.”

Blake has lived in Russia since his daring escape from a British prison in 1966 and was given the rank of Russian intelligence colonel.

Born in the Netherlands, Blake joined British intelligence during World War II. He was posted to Korea when the war there erupted in 1950 and was detained by the Communist north. He said he volunteered to work for the Soviet Union after witnessing relentless U.S. bombing of North Korea.

In a statement issued in 2017 through SVR, Blake emphasized that he decided to switch sides after seeing civilians massacred by the “American military machine.”

“I realized back then that such conflicts are deadly dangerous for the entire humankind and made the most important decision in my life — to cooperate with Soviet intelligence voluntarily and for free to help protect peace in the world,” Blake said.

As a double agent, Blake passed some of the most coveted British secrets to the Soviet Union, including a Western plan to eavesdrop on Soviet communications from an underground tunnel into East Berlin. He also exposed scores of British agents in Soviet bloc countries in Eastern Europe.

A Polish defector exposed Blake as a spy for Russia in 1961. He was convicted on spying charges in Britain and sentenced to 42 years in prison. In October 1966, he made a bold escape with help from several people he met while in custody.

Blake spent two months hiding at his assistant’s place and was then driven across Europe to East Berlin inside a wooden box attached under a car.

Blake noted in his 2017 statement that Russia has become his “second motherland,” and thanked SVR officers for their friendship and understanding. He said that Russian intelligence officers have a mission to “save the world in a situation when the danger of nuclear war and the resulting self-destruction of humankind again have been put on the agenda by irresponsible politicians.”