40 killed, more than 100 wounded in attack on Moscow's Crocus City Hall

At least 40 people were killed and over 100 others were injured in a shooting and explosion at Moscow's Crocus City Hall -- one of the biggest shopping and entertainment complexes in Russia -- Friday evening, according to the Russian Foreign Security Service.

Russia's foreign ministry called the shooting and explosion a "terrorist attack."

More than 70 ambulance teams were sent to the sight of the attack, the governor of the Moscow region reported.

Most of the building is engulfed in fire and the roof partially collapsed, Russian news agency Interfax reported. The attack was carried out by a group of up to five unknown people.

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.

In Podolsk, near Moscow, public events were canceled due to the incident at Crocus City Hall, the head of the district said. The mayor of Moscow also cancelled all "sports, cultural and other mass events," in Moscow for the weekend.

The White House is aware of the incident in Moscow but has limited information as of now, according to National Security Council Spokesperson John Kirby.

"The images are just horrible. And just hard to watch, and our thoughts, obviously, are going to be with the - the victims of this terrible, terrible shooting attack," Kirby said.

Kirby also reiterated the State Department's warning that all Americans in Moscow should avoid large gatherings, saying "they should stay put where they are, and stay plugged into the State Department for any additional updates and information."

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.