On Saturday, the Russian TASS news agency quoted a source close to the investigation as saying that Russia had detained the second scientist in the Siberian city of Novosibirsk within days, on suspicion of betraying the state.

The agency stated that Anatoly Maslov, chief scientist at the Institute of Theoretical and Applied Mechanics in Novosibirsk, about 2,800 km east of Moscow, was arrested and transferred to a prison in the Russian capital for interrogation by the Russian intelligence service.

The agency quoted the source as saying that the authorities "suspect that Maslov provided state secret data," explaining that the data was linked to hypersonic missile systems.

Russian media reported that Maslov was arrested on 28 June.

Reuters has not yet been able to verify these reports, and it has not yet been possible to contact Maslov's lawyer.

Last week, authorities arrested Dmitri Kolker, a physicist and mathematician at Novosibirsk University, on charges of treason against the state for allegedly collaborating with Chinese security services.

During the past years, the authorities arrested a number of Russian scientists and charged them with treason for allegedly transferring sensitive materials to foreigners.

The penalty for betrayal of the Russian state is imprisonment for up to 20 years.