At least 41 people were killed in two explosions that hit the Moscow metro during rush hour Monday, local news agencies reported.
The first hit the Lubyanka Metro Station in central Moscow about 7:56am local time (3:56am GMT), Russian news agency ITAR-TASS reported. Twenty-six were killed and 15 people were injured.
Sky News reported that a second explosion hit the Park Kultury station, three stations away on the same line, about 30 minutes later. AFP reported at least 15 people were killed.
RIA Novosti reported that a security source told the agency the Lubyanka station was hit by a bomb.
Interfax, quoting a Russian security source, reported the first blast may have been caused by a suicide bomber.
The Moscow Metro, which spans almost the entire Russian capital, is the world's second most heavily used metro system after Tokyo's twin subway.
The Lubyanka Station is on the Sokolnicheskaya Line of the Moscow Metro, located under Lubyanka Square.
The square houses the former headquarters of the KGB and the Lubyanka prison. It now houses the Border Guard Service of Russia, which is the nation's federal security service which succeeded the KGB.
It is located a third of a mile from the Kremlin.
Spokeswoman for the Russian emergencies ministry Irina Andrianova told ITAR-TASS 15 people were killed in the train at Lubyanka and 11 on the platform.
Sky News reported the explosion struck the second car of the train.
The Russian capital over the last decade has been hit by a string of deadly explosions claimed by militants from its turbulent southern region of Chechnya but this has become less frequent in the last years.