Vitaly I. Churkin, who had been Russia’s ambassador to the United Nations for more than a decade and championed Moscow’s perspective on barbarity in Syria, the Russian invasion of Ukraine and other global flash points, died Feb. 20 at a hospital in New York. He died a day before his 65th birthday.

Russia’s deputy U.N. ambassador, Vladimir Safronkov, said that his boss fell ill at his office. The cause wasn’t immediately known.

Mr. Churkin was the longest-serving member of the Security Council, the United Nations’ most powerful body. Among many other issues, he had recently made Russia’s views heard on the conflict in its close ally Syria, sparring with diplomats from the United States and other Western countries over whether to impose sanctions or take action to end the conflict in Syria.

Calling Mr. Churkin a “diplomatic maestro and deeply caring man,” Samantha Power, a former U.S. ambassador to the United Nations, said on Twitter that he had done all he could to bridge differences between the United States and Russia.

France’s U.N. ambassador, François Delattre, said that he and Mr. Churkin had “always worked together in a spirit of mutual respect and personal friendship,” despite their differences. One of Delattre’s predecessors, Gérard Araud — now the French ambassador to the United States — recalled Mr. Churkin as “abrasive, funny and technically impeccable.”

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Mr. Churkin, with his flawless English and unflappable demeanor, was regarded as a pinstriped survivor who had scaled the Foreign Ministry ladder over a turbulent 40-year period in his country, in part by asserting Russia’s diplomatic relevance after the Cold War.

Raised as a true believer in communism, he was ideologically flexible enough to traverse the massive political and economic changes under Soviet leader Mikhail Gorbachev and the collapse of the Soviet Union. He became a trusted troubleshooter to President Boris Yeltsin during the Balkans war that fractured Yugoslavia into bloody ethnic chaos.

During that period of the mid-1990s, he was a fixture on cable news networks and a major backstage player on forging a diplomatic solution. He used as leverage Moscow’s ties to the Bosnian Serbs, the aggressors in the Balkans war, to urge them to the negotiating table.

He was credited with brokering a deal “that averted NATO airstrikes, helped lift the siege of Sarajevo and led to the introduction of Russian troops among U.N. forces in the region,” The Washington Post reported in 1994.

“This strategy has not been without its downsides, but overall it was exactly the right thing for us to do,” he told The Post. “If we are successful at some point, if this current effort is successful, then it would be to a large extent because of that strategy. And let’s face it: This strategy assures a certain place for Russia in the negotiating effort.”

The peace did not hold. He spoke of the betrayal by the Serbs when they ignored Mr. Churkin’s demands not to press militarily on Gorazde, a Bosnian Muslim-held city that the United Nations had declared a “safe area.” He criticized the Bosnian Serbs for using “Russian support as a cover.”

Nevertheless, his work on Yugoslavia elevated his stature, and he served in increasingly visible ambassadorial posts before being named top envoy at the United Nations in 2006. He had a reputation for an acute wit and sharp repartee, especially with his American and Western counterparts.

Vitaly I. Churkin addresses members of the U.N. Security Council during a meeting on March 6, 2015. (Eduardo Munoz/Reuters)

In a 2014 interview with the New York Times, he was described as “coy and disdainful” in equal measure in discussing the Western backing of “fascist thugs” in Ukraine as well as Russian support for Bashar al-Assad amid news of the Syrian leader’s use of barrel bombs against his own people.

The diplomat pointed to the U.S. airstrikes that caused civilian deaths in Vietnam and Afghanistan, and he exuded confidence that Western capitals would ultimately need Russian leverage over Assad to end the war in Syria.

“Everybody’s speaking about barrel bombs, dropped in cities,” he told the Times. “Sounds pretty horrific. If civilians are suffering to the scale which is being described, that of course is a very dramatic thing. But we have to be clear on something: This is not something that’s per se prohibited by international law.”

Vitaly Ivanovich Churkin was born in Moscow on Feb. 21, 1952. He was a 1974 graduate of the elite Moscow State Institute of International Relations. After years as a member of the Soviet delegation to the Strategic Arms Limitation Talks, he received a doctorate in 1981 from a Soviet diplomatic academy.

He then began his ascent at the Foreign Ministry, including five years assigned to the embassy in Washington. He was a diplomat in Belgium and Canada before he was named to lead the Russian mission to the United Nations.

Mr. Churkin told Russia Today in a Feb. 7 interview that after his long career in diplomacy, the field had become much more hectic.

“Unfortunately, the world has not become more stable,” he told the Kremlin-backed news outlet.

Whatever the challenges, “the U.N. continues to be an indispensable mechanism,” Mr. Churkin said in the interview. “Without the U.N., we would be acting all on our own, without much coordination, and then we will be even less successful than we have been so far.”