Russia's combative ambassador to the United Nations, Vitaly Churkin, died suddenly in New York on Monday after being taken ill at work, the Russian Foreign Ministry said.
The ministry gave no details on the circumstances of his death but offered condolences to his relatives and said the diplomat had died one day before his 65th birthday.
It declined to comment on reports that Churkin had been taken to hospital shortly before his death.
President Vladimir Putin was "deeply upset" by the news and had greatly valued Churkin's professionalism and diplomatic talent, Russian news agencies quoted the Kremlin as saying.
The New York Post quoted unnamed sources as saying Churkin had been rushed to a Manhattan hospital from the Russian embassy after falling ill with a cardiac condition.
Tass news agency quoted Churkin's deputy Pyotr Ilyichev as saying: "The loss sustained by Russia is grave and irreplaceable.
"Ambassador Churkin remained at his work post until the last minute. He devoted his whole life to defending the interests of Russia and was to be found on the very front lines and in the most stressful posts."
Foreign ministry spokeswoman Maria Zakharova said on Facebook: "Great diplomat. Extraordinary personality. Colourful man."
Churkin was a pugnacious defender of Russian policy, notably its intensive bombing of the Syrian city of Aleppo last year to crush rebels opposed to President Bashar al-Assad.
When then-US envoy to the United Nations, Samantha Power, accused Syria, Russia and Iran last year of bearing responsibility for atrocities there, Churkin said she was forgetting the United States' own track record in the Middle East.
"The weirdest speech to me was the one by the US representative who built her statement as if she is Mother Teresa herself. Please, remember which country you represent. Please, remember the track record of your country," he said.
Farhan Haq, deputy spokesman for the UN secretary-general's office, said: "He has been such a regular presence here that I am actually quite stunned. Our thoughts go to his family, to his friends and to his government."
Churkin first came to prominence as foreign ministry spokesman for the Soviet Union from 1990 until the collapse of the superpower the following year. Despite the pressure of events, he appeared to revel in the attention of the Western correspondents who mobbed him at briefings, and was happy to respond to them at length in fluent English.
He went on to serve as deputy Russian foreign minister and ambassador to Belgium and then to Canada, eventually moving to the UN in 2006.