Anatoly Maslov is one of three scientists involved in Russia’s hypersonic missile programme that are standing trial for treason.

Anatoly Maslov’s trial opened in St Petersburg on Thursday, the first case against three hypersonic missile scientists who worked at an institute in the Siberian city of Novosibirsk and who are now facing what the Kremlin has said are “very serious accusations”.

The trial, marked as “top secret”, is closed to the media and public, the St Petersburg court has said. Maslov’s lawyer could not be reached for comment on the opening of proceedings.

A source close to Maslov,76, told the Reuters news agency that the missile scientist had suffered two heart attacks and spent time in hospital since his arrest last June in Novosibirsk.

Doctors in the pre-trial detention centre have not given Maslov access to all the medication prescribed by his normal physician, the source said, adding: “He’s holding on, but of course, his condition isn’t very good.” Reuters could not independently verify the accusation regarding the denial of medication.

Maslov was a professor and researcher at the Khristianovich Institute of Theoretical and Applied Mechanics, one of Russia’s top scientific centres.

All three defendants are specialists in hypersonics – a field of key importance to the development of Russia’s next generation of missiles, capable of flying at 10 times the speed of sound.

Colleagues at the Khristianovic Institute issued an open letter on May 15 in support of the three jailed scientists that attested to their innocence and patriotism, noting that the accused had shunned the possibility of highly paid jobs overseas to dedicate themselves to “serving Russian science”.

“We know each of them as a patriot and a decent person who is not capable of doing what the investigating authorities suspect them of,” according to a translation of the letter.

“In this situation, we are not only afraid for the fate of our colleagues. We just do not understand how to continue to do our job,” the letter’s signatories added, pointing to their fear of also being accused of treason for simply doing their jobs as scientists.

“What we are rewarded for today and set as an example to others, tomorrow becomes the reason for criminal prosecution.”

Details of the accusations against the three are classified, but the news portal of the science city where they are based said Maslov was suspected of passing secrets to China.

The source said Maslov maintains his innocence and “does not consider himself a traitor. He believes that he always did everything correctly”.

Soon after his arrest, Maslov was sent to Lefortovo prison in Moscow, a former KGB interrogation site, before being transferred to St Petersburg to stand trial.

Russia’s parliament voted in April to increase the maximum penalty for treason to life imprisonment from 20 years.