Russian media reported that at least 40 people were killed and about 100 injured in an armed attack on the Crocus City Hall in the Krasnogorsk region, located on the outskirts of the capital, Moscow.

According to local media, the attackers simultaneously opened fire in the lobby and theater of the complex, and set fire to the theater seats.

The Russian emergency service said that two to five people carried out the attack.

TASS News Agency, quoting a source in the Russian Emergencies Ministry, indicated that the patrons of the entertainment complex had been evacuated.

The agency reported that people were still trapped inside the hall designated for concerts, and that the hall caught fire, as the fire consumed a third of the building and almost the entire roof.

The Russian Federal Security Service said that there were reports of a second explosion in the concert hall.

The Moscow District Ministry of Health stated that more than 30 regional ambulance teams and 20 ambulance teams from Moscow were sent to Crocus Hall.

The Al Jazeera correspondent said that the American embassy in Moscow had warned its citizens 3 weeks ago of a major event occurring, and called on its citizens to be cautious.

Video footage circulated on Russian social media showed chaotic scenes with a large crowd of concert-goers trying to flee the hall, and then gunfire occurred.

Other footage showed a number of people lying motionless in pools of blood outside the hall.

In the wake of the attack, the mayor of Moscow announced the cancellation of all mass gatherings early next week.