UN chief Antonio Guterres offered condolences to Russia's Permanent Representative to the UN Vitaly Ivanovich Churkin, who died Monday in New York. Guterres dubbed Churkin as an "outstanding diplomat", in a statement.

Russian President Vladimir Putin esteemed Churkin's "professionalism and diplomatic talents," spokesman Dmitry Peskov said, according to the state news agency TASS.

News of Churkin's death hit the UN headquarters during a session and was met with shock. A UN official who announced the news to the session said: "He was a dear colleague of all of us, a deeply committed diplomat of his country and one of the finest people we have known." U.S. ambassador to UN, Nikki Haley said she and Churkin did not "always see things the same way, but he unquestionably advocated his country's positions with great skill". Former U.S. ambassador to UN, Samantha Power, who often argued against and criticized Churkin on critical issues, tweeted she was "devastated" by his the death of the "diplomatic maestro" who was a "deeply caring man" who did all he could to bridge differences between Washington and Moscow.

Those differences were evident when Power and Churkin spoke at the Security Council last month, when she lashed out at Russia for annexing Ukraine's Crimean Peninsula and for carrying out "a merciless military assault" in Syria. Churkin accused Democratic former President Barack Obama's administration, which Power served, of "desperately" searching for scapegoats for its failures in Iraq, Syria and Libya.

Russia's ambassador to the United Nations, a veteran diplomat known as a potent, savvy yet personable voice for his country's interests who could both spar and get along with his Western counterparts, died suddenly Monday after falling ill in his office at Russia's U.N. mission.

Vitaly Churkin was taken to a hospital, where he died a day before his 65th birthday, said Russia's deputy U.N. ambassador, Vladimir Safronkov. The cause of his death was unknown.