The US state department has instructed New York City's chief medical examiner not to disclose the cause of death of Russia's ambassador to the United Nations.
Vitaly Churkin died suddenly at work in his New York office last month.
The office of the chief medical examiner said that details of his death were being withheld to comply with international law.
Mr Churkin had served as ambassador to the UN since 2006.
The cause of the veteran diplomat's death remains unclear.
"The New York City Law Department has instructed the Office of Chief Medical Examiner to not publicly disclose the cause and manner of death of Ambassador Vitaly Churkin," a spokeswoman for the medical examiner's office, Julie Bolcer, said in a statement.
"Ambassador Churkin's diplomatic immunity survives his death," the statement, which was posted online by New York Times reporter Michael Grynbaum, added.
A state department official told the BBC that further comment or details on the results of the post-mortem could not be disclosed because of an agreement between the US and the UN.
In a letter to the city's legal office, the United States's UN mission said the Russian government had already complained about statements made by city officials regarding Mr Churkin's medical history.
Russian officials argued that the information was private and that disclosing details could cause embarrassment to the ambassador's family or hurt his reputation.
They did not specify which reports the Russian government had cited.
Last month, Russian President Vladimir Putin said he was "grieved" to learn of the death of Mr Churkin, who had developed a reputation as a steadfast defender of Russian policy.
The foreign ministry also paid tribute to an "outstanding diplomat".
State TV channels Rossiya 24 praised Mr Churkin for "putting his opponents into shock" and leaving them with "nothing to say".
Many of those "opponents" turned to social media to pay their respects to a colleague who they may have not always agreed with, but respected enormously.
Mr Churkin, who was born in Moscow, dabbled in acting as a young teenager - appearing in two films about Vladimir Lenin - before attending the prestigious Moscow Institute of International Relations, eventually beginning a career in Russia's foreign ministry.
Before taking up the position with the UN, he served as an envoy to Canada and Belgium, and as a special representative to the talks on former Yugoslavia in the early 1990s.