Russia Prime Minister Dmitry Medvedev has described the blast on the St Petersburg metro station as a "terrorist attack".
In a post on his twitter account, Mr Medvedev sent his "most sincere condolences to the families and relatives of the victims of the explosion" and said the injured would be given "all necessary assistance".
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The seven people were from Central Asian countries and investigators say they were recruiting for IS.
The bombing of a subway train in Russia has drawn condemnation from world leaders.
A blast has ripped through an underground train in Russia's second-largest city on Monday.