Shooting and explosion reported at concert hall in Moscow

A shooting followed by an explosion were reported at Moscow's Crocus City Hall -- one of the biggest shopping and entertainment complexes in Russia -- on Friday evening, according to Russian state media.

Several gunmen burst into the concert hall and opened fire with automatic weapons, state news outlet RIA Novosti reported.

Attackers then threw a grenade or incendiary bomb, starting a fire in the hall, according to Russian state media.

No official numbers on deaths or injuries have been released.

The U.S. Embassy in Moscow had issued a warning on March 7, advising U.S. citizens to avoid large gatherings for 48 hours, saying extremists have "imminent plans" to target large scale gatherings in Moscow.

This is a developing story. Check back for updates.