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

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

New York'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," US Ambassador Nikki Haley said on Tuesday, calling Churkin "brilliant, wise, gracious, and funny".

Churkin emerged as the face of a new approach to foreign affairs by the Soviet Union in 1986, when he testified before the US 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 on Monday.

AP-Reuters