ST PETERSBURG, Russia The first of three Russian hypersonic missile scientists to be arrested on suspicion of treason will go on trial next week, the court handling the case said on Wednesday.

The criminal case against Anatoly Maslov, 76, will open in St Petersburg's city court on June 1, the court said on its website.

He and two colleagues at the same Siberian institute, the Khristianovich Institute of Theoretical and Applied Mechanics (ITAM), have all been arrested on suspicion of treason in the past year.

All three 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.

The case is marked as "top secret" and will be closed to the media and public, the court said. It said Maslov's custody was extended until Nov. 10 in a closed hearing on Wednesday.

Maslov was detained last June in Novosibirsk, the largest city in Siberia and one of Russia's main science hubs. Soon afterwards he was sent to Lefortovo prison in Moscow, a former KGB interrogation site.

In St Petersburg, he has been placed in the FSB security service jail on Shalernaya Street where many Soviet dissidents were once held by the KGB, Maslov's lawyer Olga Dinze told Reuters.

She declined to comment on the case, saying that "the situation is extremely difficult."

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

Two sources have told Reuters that Alexander Shiplyuk, the director of ITAM, is suspected of passing secrets to China at a conference there in 2017. They said he denies the charge, saying the information in question was publicly available online.

ITAM has extensive international links, and says on its website that it is registered as part of Russia's military-industrial complex.

Colleagues of Maslov, Shiplyuk and the third arrested man, Valery Zvegintsev, last week published an open letter in their defence, warning that the prosecutions threaten to damage Russian science.

The Kremlin has said the three face "very serious accusations." Last month the Russian parliament voted to increase the maximum penalty for treason to life imprisonment, up from 20 years.

(This story has been refiled to add the first names of Shiplyuk and Zvegintsev in paragraphs 10 and 12)

(Reporting by Reuters reporters; Editing by Mark Trevelyan)