Russia’s ambassador to the United Nations, who collapsed in his office last month, reportedly died from a heart attack, and no foul play was suspected.

Postmortem tests were conducted on Vitaly Churkin last month. But a further examination was required, officials said.

The New York City medical examiner's said it had been asked not to publicly comment on the cause of Mr Churkin's passing by the US State Department, which cited diplomatic protocol. But the Associated Press said a city official, briefed by the medical examiner’s office, said the envoy’s death was not suspicious.

Russian Ambassador to the UN Vitaly Churkin died unexpectedly in New York (AFP/Getty Images)

The State Department asked the city in writing on February 24 to not reveal the autopsy results, because Mr Churkin's diplomatic immunity survives his death.

“The United States insists on the dignified handling of the remains of our diplomatic personnel who pass away abroad (including in Russia) and works to prevent unnecessary disclosures regarding the circumstances of their deaths,” wrote James Donovan, minister for host country affairs for the US mission to the United Nations.

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The AP said that in a follow-up letter on March 1, the State Department said the Russian Federation raised concerns after the autopsy had been conducted, and “voluntary statements reported in the media about Ambassador Churkin's medical history” prompted complaints from Russian diplomats.

“The information reported was very private in nature and included information about which even they had no knowledge,” said Mr Donovan. office.

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A spokesman for Russia's UN Mission said: “We regard the decision of the New York City Law Department as the one that fully complies with the principles of inviolability of private life and diplomatic immunity.”

The medical examiner is responsible for investigating deaths that occur by criminal violence, accident, suicide, suddenly or when the person seemed healthy, or if someone died in any unusual or suspicious manner. City policy is to publicly release the cause of death.

Mr Churkin, who died on February 20 at a hospital at the age 64, had been Russia’s envoy at the UN since 2006. He was the longest-serving ambassador on the Security Council, the UN’s most powerful body.