Vitaly Churkin, who has died aged 64 after suffering a heart attack, was the longest serving Russian ambassador to the United Nations, a tenacious, committed and often witty defender of his country’s interests with a career that ranged from nuclear disarmament to the Balkan wars. He was appointed to the UN post in 2006 shortly before western governments stepped up their efforts to expand Nato into the highly sensitive arena of Ukraine and Georgia, souring the atmosphere of western co-operation with the Kremlin that had marked the early post-Soviet period under Boris Yeltsin and the first years of Vladimir Putin.

Like Sergei Lavrov, the Russian foreign minister, who also served for 10 years in New York as Russia’s UN representative, Churkin epitomised the peak of professionalism in his country’s diplomatic service, mastering the detailed dossiers of every conflict and crisis around the world while explaining Russia’s stance with clarity, firmness and dignity.

He hated the moralising tone of his US, British and French counterparts on the UN security council who, he felt, were not only hypocritical but were playing to the global gallery and aiming to score rhetorical points instead of looking for compromises that could lead to the resolution of differences. This applied particularly to the war in Syria, about which western governments tabled resolutions that could lead, in the Russian view, to full-scale military intervention against the Syrian government and which they knew Churkin was bound to veto. Russia preferred to produce resolutions that criticised the Syrian army for using “disproportionate” force and sought agreement on ceasefires. Churkin consulted the security council’s five permanent members on these resolutions, but chose not to provoke vetoes when he realised there was no consensus.

Churkin’s rhetorical fire was directed especially at Samantha Power, his US counterpart, who had no previous diplomatic experience before she took up the post. He once accused her of making a “strange” speech as though she were Mother Teresa, a role that the US record of intervention in the Middle East did not justify. When she condemned Russian bombing in Aleppo, he responded by recalling US carpet-bombing of Vietnam. Two weeks before his death he went on the diplomatic attack when the British UN ambassador, Matthew Rycroft, called on Moscow to “return Crimea” to Ukraine. Churkin said: “Return the Malvinas, return Gibraltar. Then maybe your conscience might be clearer and you could discuss other topics.”

Challenged over civilian casualties caused by Russian bombing in Aleppo by the former British diplomat Stephen O’Brien, who had become the UN’s relief co-ordinator, Churkin said: “If we wanted to hear a sermon, we would go to church. If we wanted to hear poetry, we would go to a theatre. Give us one fact, please, or leave this kind of storytelling for the novel you may well write later.”

While often sharp with western representatives in the cockpit of the security council, Churkin also had enough confidence and Kremlin backing to criticise Russia’s allies in public. A few months after Russia started to use its air force in Syria, President Bashar al-Assad told an interviewer in February 2016 that he aimed to win back all of Syria from opposition forces. Churkin promptly responded in the Russian newspaper Kommersant with a politely worded but clear rebuke: “I heard President Assad’s remarks on television ... Of course they do not chime with the diplomatic efforts that Russia is undertaking … The discussions are about a ceasefire, a cessation of hostilities in the foreseeable future. Work is under way on this. Russia has invested very seriously in this crisis, politically, diplomatically, and now also in the military sense. So of course we would like Bashar al-Assad to take account of that.”

Born in Moscow, Churkin showed considerable acting talent as a child. He starred as an 11-year-old in a film about Lenin and in two other films over the next three years. After he left school he attended the prestigious Moscow State Institute for International Relations, the main training ground for prospective high flyers in the Soviet foreign service.

On graduation in 1974 he spent the next five years negotiating with the Americans as a diplomat in the Soviet team at the Strategic Arms Limitation Treaty (Salt) talks, a post that required him to master a huge amount of technical detail on nuclear weapons. After three years in Moscow as a third secretary on the US desk in the foreign ministry, he was posted to Washington in 1982.

There he made history in 1986 by becoming the first Soviet official to testify before a Congressional committee. The Soviet ambassador, Anatoly Dobrynin, chose his junior colleague to parry questions on the Chernobyl nuclear power station disaster because of his almost flawless English and his diplomatic charm – as well as his technical expertise. He returned to Moscow in 1987 shortly before Mikhail Gorbachev’s “revolution from above” became a “revolution from below”, with nationalist pressures developing in the Baltic states and the Caucasus, anti-communist street protests becoming an everyday affair in many Soviet cities, and economic problems mounting.

Churkin worked in the international department of the Soviet Communist party’s central committee and in 1990 was appointed to lead the foreign ministry’s information department. Here he first began to shine as a user of dry wit in verbal combat with westerners, treating foreign journalists with nuanced explanations of the twists and turns of Gorbachev’s foreign policy.

During the Bosnian war, with the Russian Federation emerging from the ruins of the Soviet Union, Churkin served as Moscow’s representative to the sputtering talks process from 1992 to 1994. He used Russia’s traditional links with the Orthodox Serbs to broker a deal that was to lift the Bosnian Serbs’ siege of Sarajevo and lead to Russian troops joining a UN peacekeeping force. When the Bosnian Serbs broke it by advancing on the Muslim-held safe haven of Goražde, Churkin accused them of exploiting “Russian support as a cover”.

Between 1994 and 1998 he served as Russia’s liaison with Nato when the first moves – opposed by many former US and British ambassadors to Moscow – were made to expand Nato to the Russian border by bringing former Soviet allies, such as Poland and Hungary, into the US-led alliance.

From 1998 he spent five years in Ottawa as Russian ambassador to Canada, followed by three years as a roving ambassador dealing with Arctic negotiations with Europe and the US.

In 2006 he attained what is considered the top post in the Russian foreign ministry (apart from foreign minister), the highly stressful role of representing his country at the UN. He performed it with dignity and diligence until dying of a cardiac arrest at his desk. He had a history of heart problems and also suffered from leukaemia, which he did not reveal to his colleagues. After more than a decade at the UN, he joked that he was becoming the “permanent” Russian representative.

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

• Vitaly Ivanovich Churkin, diplomat, born 21 February 1952; died 20 February 2017