Russia's Federal Security Service (FSB) says it has detained five members of an alleged Islamic State cell planning attacks in several regions, and seized an array of their weapons, Russian news agencies reported.
The five suspects were apprehended in the city of Yaroslavl, northeast of Moscow, on May 3 and 4, Interfax and TASS reported late on May 4, quoting an FSB statement.
Russia, whose military is backing the Syrian government in its war against rebel forces, including Islamic State militants, is hosting the soccer World Cup next month and has been increasing security measures ahead of the tournament.
On April 27, the FSB said it had thwarted an alleged Islamic State plot to carry out a series of high-profile attacks in the Moscow area, where soccer World Cup matches are due to take place this summer.
The FSB did not reveal the identities of those it detained on May 4, or provide details on when or where they were planning to stage attacks.
"In the course of house searches, an arsenal of home-made explosive devices and substances, firearms, and ammunition were seized from the detained," the FSB was quoted as saying.
The FSB claimed the suspects' activities were being coordinated partly from abroad using the Telegram instant messaging service, which Russia moved to block last month.
State communications regulator Roskomnadzor began blocking access to Telegram on April 16 after it refused to comply with a court order to grant security services access to its user-encrypted messages.
The FSB has said it needs access to some user messages for its work, although supporters of Telegram -- thousands of whom rallied against the ban in Moscow this week -- say the move amounts to an attack on Internet freedom.