Tom Brokaw of NBC News, who had known Mr. Churkin for many years, recalled that at a lunch at his home in 2015, the ambassador described Mr. Putin’s administration as “a kleptocracy.”

“He was privately very critical of Putin and how he was running the country,” Mr. Brokaw said on Monday. “He was equally critical of the Obama administration. He said: ‘They just don’t get us. They’re not dealing with us the right way.’”

Russia’s relations with the United States soured during Mr. Churkin’s tenure at the United Nations, first over Libya and then over crises in Syria and Ukraine. In an interview in October, he said that Russian-American relations had not been so strained since the Arab-Israeli conflict nearly brought the two Cold War powers to a military confrontation four decades ago.

He was a staunch defender of the Syrian government of Bashar al-Assad. He used Russia’s veto at the Security Council to block six resolutions that would have punished the Assad government, including one aimed at referring its actions to the International Criminal Court.

He repeatedly questioned all evidence that appeared to point to Syrian or Russian government crimes in Syria and met every Western criticism of Russia’s conduct there with retorts about the West’s role in Yemen and elsewhere. He picked apart every word of every draft resolution to ensure that it satisfied Russian interests.

But he could also surprise his fellow diplomats. Last December, he negotiated with his French and American colleagues for three hours in a closed room on the wording of a draft resolution to send United Nations monitors to oversee evacuations from Aleppo. It was the first time in months that the Security Council had reached a consensus on Syria.