Three Russian academics who have worked on hypersonic missile technology face "very serious accusations", the Kremlin said on Wednesday, in a treason investigation that has spread alarm through Russia's scientific community.
Kremlin spokesman Dmitry Peskov said he was aware of an open letter from Siberian scientists in defence of the men, but that the case was a matter for the security services.
In the letter, published on Monday, colleagues of Anatoly Maslov, Alexander Shiplyuk and Valery Zvegintsev protested their innocence and said the prosecutions threatened to inflict grave damage on 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," they said.
President Vladimir Putin has boasted that Russia is the global leader in hypersonic missiles, capable of travelling at speeds of up to Mach 10 (12,250 kph) to evade enemy air defences. On Tuesday, Ukraine said it had managed to destroy six of the weapons in a single night, although Russia disputed this.
Notices of academic conferences stretching back over many years show the arrested scientists were frequent participants.
In 2012, Maslov and Shiplyuk presented the results of an experiment on hypersonic missile design at a seminar in Tours, France. In 2016, all three were among the authors of a book chapter entitled "Hypersonic Short-Duration Facilities for Aerodynamic Research at ITAM, Russia".
The open letter from their colleagues at ITAM - the Khristianovich Institute of Theoretical and Applied Mechanics in Novosibirsk - said the materials the scientists had presented in international forums had been checked repeatedly to ensure they did not include restricted information.
The cases showed that "any article or report can lead to accusations of high treason", the open letter said.
"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 also cited the case of Dmitry Kolker, another Siberian scientist who was arrested last year on suspicion of state treason and flown to Moscow despite suffering from advanced pancreatic cancer. Kolker, a laser specialist, died two days later.
It said such cases were having a chilling effect on young Russian scientists.
"Even now, the best students refuse to come to work with us, and our best young employees are leaving science. A number of research areas that are critically important to laying the fundamental groundwork for the aerospace technology of the future are simply closing because employees are afraid to engage in such research."
Asked about the letter, Kremlin spokesman Peskov said: "We have indeed seen this appeal, but Russian special services are working on this. They are doing their job. These are very serious accusations."