President Trump on Tuesday mourned the death of Russia’s ambassador to the United Nations, who passed away suddenly a day earlier.

Trump said he was “saddened” to learn of the death of Vitaly Churkin, whom he described as an “accomplished diplomat” who served his country for more than a decade.

“While American officials sometimes disagreed with their Russian counterparts, Ambassador Churkin played a crucial role in working with the United States on a number of key issues to advance global security,” he said in a statement.

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Churkin died in a New York hospital on Monday, one day short of his 65th birthday. A law enforcement official told CNN he died of cardiac arrest.

Trump is facing scrutiny for his team’s alleged ties to Russia, leading many critics to question whether he will stand up to Moscow in hot spots such as Ukraine and Syria.

Trump's national security adviser, Michael Flynn, last Monday resigned after it was revealed he misled Vice President Pence and others about whether he discussed U.S. sanctions with another Russian envoy before Trump's inauguration.