United Nations, US: Russia’s ambassador to the United Nations, Vitaly Churkin, who for years fended off Western criticism and defended Moscow’s actions in Ukraine and Syria, died Monday in New York. He was 64.

Churkin collapsed while at work at the Russian mission to the United Nations Monday morning and was rushed to a Manhattan hospital, apparently suffering from heart problems, diplomatic sources said.

In a statement announcing his death, the foreign ministry in Moscow described him as an ‘outstanding diplomat.’ There was no information on the cause of death.

Kremlin spokesman Dmitry Peskov said President Vladimir Putin ‘highly valued the professionalism and diplomatic talent of Vitaly Churkin’ and offered condolences, Interfax reported.

Russian foreign ministry spokeswoman Maria Zakharova paid tribute to Churkin, writing on Facebook: ‘A great diplomat. An extraordinary personality. An outstanding individual. We have lost someone very dear.’

News of his death was met with shock at UN headquarters where Churkin, who would have turned 65 on Tuesday, had been a towering presence for a decade.

UN Secretary General Antonio Guterres said Churkin had served ‘with distinction through some of the most challenging and momentous periods of recent history.’

Guterres described him as a ‘forceful presence on the Security Council,’ a ‘uniquely skilled diplomat, a powerful orator with great wit and a man of many talents and interests.’

Diplomats at a UN meeting observed a moment of silence and a tribute to him was planned at the Security Council on Tuesday.

Russian Deputy UN Ambassador Petr Iliichev said Churkin was at work ‘until his last moments.’

‘His whole life was dedicated to defending the interests of Russia,’ Iliichev said. ‘He was on the frontline.’

‘We’ve lost a prominent diplomat, a strong negotiator, a wonderful individual, a teacher,’ said Iliichev.

UN spokesman Farhan Haq, who heard the news during the regular daily briefing, offered his condolences, adding: ‘He has been such a regular presence here that I’m actually quite stunned.’

Western rift with Russia

A career diplomat born in Moscow who studied English from a young age, Churkin had been UN ambassador since April 2006, after serving as Russian ambassador to Canada and to Belgium.

During that decade, relations between Russia and the West soured, culminating with a major rift over Moscow’s support for separatists in east Ukraine and its military intervention in Syria.

In the final months, Churkin clashed repeatedly with then-US ambassador Samantha Power over the bloodshed in Aleppo, staunchly defending Moscow’s view that the Syrian government was waging all-out war against ‘terrorists.’

Russia resorted to its veto six times to block action on Syria.

Power tweeted that she was ‘devastated’ and described Churkin as a ‘diplomatic maestro’ and ‘deeply caring man’ who did all he could to bridge US-Russian differences.

Power’s successor, Nikki Haley, who arrived less than a month ago, said Churkin had ‘showed himself to be a gracious colleague’.

‘We did not always see things the same way, but he unquestionably advocated his country’s positions with great skill,’ said Haley in a statement.

French Ambassador Francois Delattre said Churkin was ‘one of the most talented diplomats’ he’d ever met and ‘an exceptional representative of Russia’ at the UN.

British Ambassador Matthew Rycroft tweeted that he was ‘absolutely devastated,’ describing Churkin as ‘a diplomatic giant & wonderful character.’

Churkin had served as foreign ministry spokesman in the early 1990s and also as special envoy to the former Yugoslavia.

Growing up in Moscow, he dabbled in acting in his early teens, notably in two films about Soviet founding father Vladimir Lenin.

He graduated from the prestigious Moscow Institute of International Relations and began his career at the foreign ministry as a translator before working at the Soviet embassy in Washington in the 1980s.

Given his long posting at the UN—previously held for a decade by Russia’s Foreign Minister Sergei Lavrov—Churkin joked in a recent interview that his stint was surpassed only by the envoy from Turkmenistan.

He is survived by his wife Irina, son Maxim and daughter Anastasiya.