Russian authorities ordered security forces on high alert on Tuesday after receiving information from foreign officials pointing to the threat of a terrorist attack on public transportation, officials said.
Authorities are checking information about the potential threat, they said.
A federal anti-terrorism headquarters received information "from foreign partners ...
About the possibility a subversive terrorist act could be committed on ground transport and in the metro," according to a statement confirmed by a Federal Security Service official who said he was not authorised to give his name for publication.
Federal Security Service chief Nikolai Patrushev, who also heads the anti-terror centre, ordered anti-terror forces on high alert and called for stepped-up measures to prevent any attack, the statement said.
It offered no details on the nature of the threat or the measures to be taken.
It was unclear what country the information about the potential threat came from; Russia cooperates against terrorism with countries around the world, including the United States.
The statement did not say whether the threat was limited to any particular location in Russia, where several cities have subway systems.