Vitaly Churkin was taken to a hospital, where he died Monday, according to Russia's deputy U.N. ambassador, Vladimir Safronkov. Churkin would have turned 65 Tuesday. The cause of his death was not immediately known.

Churkin had been Russia's envoy at the United Nations since 2006. He was the longest-serving ambassador on the Security Council, the U.N.'s most powerful body.

U.N. Secretary-General Antonio Guterres called him "a uniquely skilled diplomat, a powerful orator with great wit, and a man of many talents and interests."

"Although we served together for a short time, I greatly appreciated the opportunity to work with him and will deeply miss his insights, skills and friendship," Guterres said in a statement.