"The Federal Security Service detained Ivan Safronov, adviser to the director general of Roscosmos state corporation, in Moscow," an FSB spokesperson told Russian state news agency TASS.
"Safronov was performing tasks for one of NATO's intelligence services, collecting state confidential data about Russia's military and technical cooperation, defense and security and handing it over to its [NATO intelligence service] representative."
Roscosmos released a statement announcing Safronov had been detained on suspicion of treason and that the agency was providing assistance to the "investigating authorities."
The statement added Safronov's detention was unconnected to his current work with the corporation.
Safronov's lawyer Ivan Pavlov told Russian media on Tuesday the charges "obviously" stemmed from his work as a reporter.
However, President Vladimir Putin's spokesperson appeared to take a different line in a conference call on Wednesday. "I rule it out," said Dmitry Peskov, when asked if he was misled by the special services that the "Safronov case" was unrelated to Safronov's journalism.
"As before, we proceed from the fact that this is not connected with the journalistic activities of Safronov."
Roscosmos head Dmitry Rogozin told TASS Tuesday that Safronov had no access to classified information during his time at the space agency.
"He was invited to cover Roscosmos activities one and a half months ago. He did not have classified information clearance," Rogozin said.
Safronov was appointed in May to be an adviser to Rogozin, who said he had known his aide for a couple of years while he was still a journalist. "I did not doubt his high professionalism and personal decency," Rogozin added.
Safronov previously worked as a special correspondent for Russia's Kommersant and Vedomosti newspapers covering military, defense and space industries.
Kommersant released a statement defending Safronov's professionalism and describing the accusation of treason as "absurd."
"Ivan is not called one of the best journalists in the country in vain," the newspaper said.
"We know not only about his highest professionalism, but also about his character. Ivan is a true patriot of Russia, who wrote about the army and space because he sincerely cared about them. Suspicion of treason in his case seems absurd."
Pavlov said little evidence had been submitted to support the charges. "There are no documents testifying that Ivan transferred any information to foreign citizens," he said.
Safronov will remain in custody for two months according to Russian state news agencies RIA-Novosti and TASS. The FSB plans to bring high treason charges against Safronov on July 13, Pavlov told CNN on Wednesday.
"On July 13, he will be charged in the investigator's office, and the next hearing will be a consideration of our appeal, which we just filed today and it will be examined by the Moscow City Court," Pavlov added.
If found guilty, Safronov faces up to 20 years in jail.
According to the Russian monitoring group OVD-info, at least 28 people -- some of them journalists -- have been detained in Moscow for protesting against Safronov's arrest.