Medical examiners who performed an autopsy on Russia's ambassador to the United Nations said Tuesday that more tests are needed to determine how and why he fell ill in his office and later died.



Vitaly Churkin, who died Monday, a day before his 65th birthday, had been Russia's envoy at the U.N. since 2006. He was the longest-serving ambassador on the Security Council, the U.N.'s most powerful body.



New York City's medical examiners concluded Churkin's death needed further study, which usually includes toxicology and other screenings. They can take weeks.



The medical examiner is responsible for investigating deaths that occur by criminal violence, by accident, by suicide, suddenly or when the person seemed healthy, or in any unusual or suspicious manner. Most of the deaths investigated by the office are not suspicious.



Churkin's case was referred to the medical examiner's office by the hospital, spokeswoman Julie Bolcer said.



Russian President Vladimir Putin esteemed Churkin's "professionalism and diplomatic talents," spokesman Dmitry Peskov said, according to the state news agency TASS. Moscow has not yet given a date for Churkin's funeral.



Diplomatic colleagues from around the world mourned Churkin as a master in their field, saying he was deeply knowledgeable about diplomacy and dedicated to his country while also being a personable and witty colleague.



"He could spot even the narrowest opportunities to find a compromise," U.S. Ambassador Nikki Haley said Tuesday, calling Churkin "brilliant, wise, gracious, and funny."



Her predecessor, Samantha Power, described him on Twitter as a "diplomatic maestro and deeply caring man" who had done all he could to bridge differences between the U.S. and Russia.



Those differences were evident when Power and Churkin spoke at the Security Council last month, and Power lashed out at Russia for annexing Ukraine's Crimean Peninsula and for carrying out "a merciless military assault" in Syria. Churkin countered that Democratic former President Barack Obama's administration, in which Power served, was "desperately" searching for scapegoats for its failures in Iraq, Syria and Libya.



Churkin died weeks into some major adjustments for Russia, the U.N. and the international community, with a new secretary-general at the world body and a new administration in Washington. Meanwhile, the Security Council discussed Ukraine on Tuesday and is set to discuss Syria later in the week.



Ukraine is currently holding the Security Council's rotating presidency, and Ukrainian Ambassador Volodymyr Yelchenko led members Tuesday in a moment of silence in Churkin's memory. Yelchenko didn't add his own words to the tributes that followed, though Ukrainian Foreign Minister Pavlo Klimkin gave condolences when reporters asked afterward.



While noting "fundamental differences" with Churkin's political positions and how he presented them, that divide "does not put us away from our point of expressing human condolences," Klimkin said.



From Moscow's vantage point, "Churkin was like a rock against which were broken the attempts by our enemies to undermine what constitutes the glory of Russia," TASS quoted Deputy Foreign Minister Sergei Ryabkov as saying.



Churkin's U.N. counterparts "experienced and respected the pride that he took in serving his country and the passion and, at times, very stern resolution that he brought to his job," said General Assembly President Peter Thomson, of Fiji. But colleagues also respected Churkin's intellect, diplomatic skills, good humor and consideration for others, said Thomson, who called for a moment of silence at a meeting Monday.



Churkin emerged as the face of a new approach to foreign affairs by the Soviet Union in 1986, when he testified before the U.S. Congress about the Chernobyl nuclear plant disaster. His fluent English, fashionable appearance and friendly, sometimes humorous, exchange with lawmakers marked a departure from the tone lawmakers had come to expect from the Soviet Union.



After he returned to the foreign ministry in Moscow, he ably parried with Western correspondents at briefings in the early 1990s. He later held ambassadorships in Canada and Belgium, among other posts.



Churkin told Russia Today in an interview this month that diplomacy had become "much more hectic," with political tensions rising and stability elusive in various hotspots. At the time, he looked in good health, reporter Alexey Yaroshevsky tweeted Monday.