The FSB claims, according to Russian news agencies, that the suspects planned to murder a high-ranking military officer using a remotely controlled car bomb and that another high-ranking military officer would be murdered using an explosive charge hidden in an envelope.
The security service does not name the alleged targets.
Last week, Russian Lieutenant General Igor Kirillov and one of his colleagues were killed in a bombing in Moscow, which Ukraine has taken responsibility for.
Kirillov was the highest-ranking chief of Russia's radiological, chemical, and biological defense and was accused by Ukraine of using chemical weapons against Ukrainian soldiers.
The day after the attack, the FSB arrested a suspect, a citizen of Central Asian Uzbekistan, who claimed to have been recruited by Ukrainian intelligence services.
