In a statement on Monday, the Russian foreign ministry said the country's ambassador to the United Nations, Vitaly Churkin, had "unexpectedly" died in New York.
"The outstanding Russian diplomat passed away at his work post," the statement 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.
Churkin has been Russia's envoy at the United Nations for a little over a decade and was considered Moscow's great champion at the UN. Previously he worked at the foreign ministry in Moscow, served as an envoy to Canada (1998-2003), Belgium (1994-1998) and as a special representative to the talks on former Yugoslavia (1992-1994).
'Pillar of the UNSC'
Tributes to the ambassador soon followed the announcement of his death. The Kremlin's spokesman Dmitry Peskov said Russian President Vladimir Putin was grieving after learning of Churkin's death.
"The head of state highly valued Churkin's professionalism and diplomatic talent," Peskov said, adding that the president had expressed his condolences to Churkin's loved ones.
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."
Meanwhile, the UK Mission to the United Nations tweeted: "Vitaly Churkin was a pillar of the [UN Security Council] for over a decade. Our deepest condolences to his family [and] colleagues..."
Churkin was a pugnacious defender of Russian policy, most notably its intensive bombing of the Syrian city of Aleppo last year to crush rebels opposing Syrian President Bashar al-Assad.
When then-US envoy to the UN Samantha Power accused Syria, Russia and Iran last year of bearing responsibility for atrocities there, Churkin said she was acting like Mother Teresa and forgetting her own country's track record in the Middle East.
On hearing of her counterpart's death, Power said was "devastated," describing Churkin as a "diplomatic maestro." He was a "deeply caring man" who tried to bridge differences between the US and Russia, she added.
Power's successor Nikki Haley - who took up the post last month - also offered her condolences, saying that Churkin "showed himself to be a gracious colleague."
"We did not always see things the same way," she added. "But he unquestionably advocated his country's positions with great skill."
ksb/msh (dpa, AFP, Reuters)