A pilot of a UTair Boeing 757 flying from Bangkok to Novosibirsk in Russia's Siberia region died as the plane was flying over China, officials said.
Sergei Golev, 44, sustained a heart attack in the cockpit and died despite all attempts at resuscitation, a local prosecutor's office spokeswoman said.
Uralinform.ru reported that a cardiologist found among the passengers also failed to revive him.
The chief pilot attempted an emergency landing in the Chinese city of Chengdu, but abandoned it because Golev died before the jet touched down.
The incident took place on Tuesday, but was not reported before.
Some media reports said the Boeing was bound for Moscow and executed an emergency landing in Novosibirsk, and that Golev, though a pilot, was flying as a passenger.
But a spokeswoman for the regional transportation prosecutors told Komsomolskaya Pravda newspaper that the plane was flying to Novosibirsk and that Golev was a crew member - a reserve pilot tasked, ironically, with replacing a crew member in case they fall ill.