The two suspects behind a deadly shooting at Moscow's Crocus City Hall have fronted court, where they face possible life imprisonment following the shooting deaths of at least 137 people.

Dalerdhon Mirzoyev and Saidakrami Rachabalizoda have been identified as alleged perpetrators of the attack, both with suspected links to the Islamic State group.

The group has released a video following the deadly shooting, appearing to show concertgoers being gunned down in the lobby of the venue.

The death toll has risen to 137 with another 154 injured, making it the worst terror attack in Russia in two decades.

A nationwide day of mourning was observed in Russia on Sunday, with thousands leaving flowers and tributes at the severely damaged venue which is still being searched by emergency crews.

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