The UN observed a moment of silence on Monday to honor Russia's ambassador to the United Nations, Vitaly Churkin, who died suddenly after falling ill in his office at their mission, officials said.

Vitaly Churkin, 64, was taken to a hospital in New York City just after 9.30am, where he passed away, Russia's deputy UN ambassador, Vladimir Safronkov said.

According to sources at the NYPD he suffered a fatal heart attack.

President Vladimir Putin was 'deeply upset' by the news and had greatly valued Churkin's professionalism and diplomatic talent, Russian news agency TASS quoted the Kremlin as saying.

'The president was grieved to learn about the death of Vitaly Churkin,' spokesman Dmitry Peskov said, according to the agency.

Churkin had been Russia's envoy at the United Nations since 2006 and was considered Moscow's great champion at the UN.

Diplomatic colleagues from around the world mourned Churkin as a powerful and passionate voice for his nation, with both a deep knowledge of diplomacy and a large and colorful personality.

He was the longest-serving member of the Security Council, the UN's most powerful body.

Passed away: Vitaly Churkin died at the Russian embassy on 67th Street at around 9.30am on Monday morning

Scene: New York City police officers and a patrol car are stationed outside the Russian Mission to the United Nations with flags flying at half mast on Monday

Distress: Russian President Vladimir Putin, Foreign Minister Sergey Lavrov (center), and Russian ambassador to the United Nations Vitaly Churkin (left) listen during a meeting in the Kremlin in Moscow in November of last year

The former child-actor was due to celebrate his 65th birthday on Tuesday and police sources have already said that no foul play is suspected.

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.'

Russian foreign ministry spokeswoman Maria Zakharova said on Facebook: 'Great diplomat. Extraordinary personality. Colorful man.'

His diplomatic counterparts weighed in with tributes.

'In my short time at the United Nations, Ambassador Vitaly Churkin showed himself to be a gracious colleague,' the US ambassador to the world body, Nikki Haley, said in a statement.

'We did not always see things the same way, but he unquestionably advocated his country's positions with great skill. We send our prayers and heartfelt condolences to lift up his family and to the Russian people,' she said.

Stand to attention: General Assembly members observe a moment of silence for Vitaly Churkin, Russia's ambassador to the United Nations, during an informal briefing of the UN General Assembly

Address: United Nations General Assembly President Peter Thomson, center, speaks about the death of Vitaly Churkin, Russia's ambassador to the U.N., and announces a moment of silence

Honored: The official Twitter feed for the Russian Federation posted this tribute to Churkin (to die in harness is a reference to the fact that he was working up until his death)

The Russian made headlines last year when he filed a complaint to the UN in September demanding that a high profile human rights official stop criticizing then-presidential candidate Donald Trump.

Churkin sought a private audience with Secretary General Ban Ki-moon in to protest speeches by Zeid Ra'ad Al Hussein, the UN commissioner, that appeared to label Trump a 'demagogue'.

'Prince Zeid is overstepping his limits from time to time and we're unhappy about it,' said Churkin at the time.

The ambassador's death comes as speculation intensifies around Washington as to the Trump administration's links, if any with Russia and President Putin.

Last week, National Security Adviser Michael Flynn resigned after it emerged he lied to Vice President Pence about alleged assurances he gave to Russia regarding the lifting of US sanctions.

Churkin's death came at the start of a week when the Security Council is expected to discuss Ukraine and Syria.

The US and the European Union imposed sanctions on Russia for its 2014 annexation of Ukraine's Crimean Peninsula and its support for insurgents in eastern Ukraine.

Meanwhile, the United States, Britain and France have been pressing the UN Security Council to impose sanctions on the Syrian government for using chemical weapons, while Russia - Syria's closest ally - has repeatedly questioned investigators' conclusions linking chemical attacks to Syrian President Bashar Assad's government, which has denied them.

Negotiating: Russian Ambassador to the UN Vitaly Churkin speaks at the UN Security Council on a Russian-Turkish peace plan for Syria, on December 31, 2016, at the UN Headquarters in New York

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.

Despite his colorful reputation and personal popularity, Churkin presided over the deepest souring of US and Russian relations since the 1970s.

From his defense of Russia's invasion of Georgia in 2006, to the annexation of the Crimean peninsula and support for separatists in Ukraine, Churkin vetoed 15 UN resolutions as Russian envoy.

In an interview he gave recently to CBS News, Churkin admitted that the US and Russia had slipped back to a Cold War climate, the 'worst since 1973' and said, 'it is fashionable to blame Russia for everything.'

When then-U.S. 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 U.S. 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.

Shortly after his death was announced, Power posted a heartfelt message on Twitter, writing, 'Devastated by passing of Russian UN Amb Vitaly Churkin. Diplomatic maestro & deeply caring man who did all he could to bridge US-Russian differences.'

Tragedy at the United Nation: The Dutch ambassador to the UN posted this picture of the moment they were told Churkin had passed away

Tribute: Samantha Power who was the US ambassador to the UN under Barack Obama posted this to Twitter shortly after being told the news

The British ambassador to the UN, Matthew Rycroft, said that the world had lost a 'diplomatic giant'.

Francois Delattre, the French Ambassador to the UN told Bloomberg, 'He was one of the most talented diplomats that I've worked with, a tough negotiator. 'He was an excellent representative of his country.'

Karel van Osterom, the Netherlands' ambassador to the UN took to Twitter and posted a tribute to the career diplomat, 'A brilliant ambassador who served his country & people. May he rest in peace.'

He also tweeted a picture from inside the UN of the moment when diplomats were told of his passing.

Farhan Haq, deputy spokesman for the U.N. 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.

Respect: President Obama's National Security Advisor and former US ambassador to the UN Susan Rice also paid Churkin a warm compliment on his unexpected death

Warm: The British ambassador to the UN, Matthew Rycroft posted his condolences to Twitter

Long hours: Churkin listens intently during the debate into the Russian-Turkish peace plan at the UN Security Council in New York City last 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.

According to Bloomberg, he possessed a witty repartee and was popular among his American and Western counterparts.

Before arriving in New York City in 2006, Churkin was the Russian ambassador to Canada, Belgium in the 1990s and a liason ambassador to NATO.

Prior to this Churkin was previously ambassador at large and earlier served as the foreign ministry spokesman. Churkin had a doctorate in history and was a graduate of the Moscow State Institute of International Relations.

Churkin was previously ambassador at large and earlier served as the foreign ministry spokesman. Churkin had a doctorate in history and was a graduate of the Moscow State Institute of International Relations.