Dozens dead after gunmen open fire at Moscow concert hall ...

The Federal Security Service, Russia's top security agency, said there are dozens dead or wounded in a Moscow concert hall attack, Russian state news agencies reported Friday night, after several gunmen in combat fatigues burst into the venue and fired automatic weapons at the crowd.

Russian media outlets reported that between two to five assailants were involved in the attack and also used explosives, causing a massive blaze at the Crocus City Hall on the western edge of Moscow.

The attack took place as crowds gathered for a concert by Picnic, a famed Russian rock band, at the hall, which can accommodate over 6,000 people.

Russian news reports said visitors were being evacuated, but some said an unspecified number of people could have been trapped by the blaze.

Fire engulfed a third of the venue building, and its roof is almost completely engulfed in flames, state news agency TASS reported, adding that helicopters had been called in to help.

Shortly before 11:00 p.m. local time, Russia's Interfax news agency reported firefighting crews had contained the fire.

Extended rounds of gunfire could be heard on multiple videos posted by Russian media and Telegram channels.

More videos showed up to four attackers, armed with assault rifles and wearing caps, who were shooting screaming people at point-blank range.

Another one showed a man inside the auditorium, saying the assailants set it on fire, with incessant gunshots ringing out in the background.

WATCH | Social media video captures sounds of gunshots, panic as Russian concert hall attacked: 

"Suddenly there were bangs behind us — shots. A burst of firing, I do not know what," one witness who asked not to be named told Reuters.

"A stampede began, everyone ran to the escalator," the witness said. "Everyone was screaming, everyone was running."

Russian media reports said that riot police units were being sent to the area as people were being evacuated.

Special units of Russia's National Guard have arrived on the scene of a shooting at a concert venue near Moscow on Friday, TASS said.

More than 70 ambulance crews have been sent to the scene, the RIA news agency reported.

There were no immediate claims of responsibility for the raid, but Interfax reported the Russian National Guard is searching for those who carried out the attack — the worst in Russia in two decades.

Russia's Federal Security Service said it is taking "all necessary measures" in connection with the shooting, Russian news agency Interfax reported on Friday.

Russian President Vladimir Putin is receiving regular updates about the attack, Kremlin spokesperson Dmitry Peskov said on Friday.

Putin had given all necessary instructions in connection with the incident, Russian news agencies reported Peskov saying.

Russian ombudsperson Tatyana Moskalkova labelled the incident a "terrorist attack," while Foreign Ministry spokesperson Maria Zakharova said it was "a monstrous crime" and called on the international community to condemn the attack.

Moscow Mayor Sergei Sobyanin described the attack as a "huge tragedy."

He cancelled all mass gatherings in the city for this weekend.

Russian authorities said they had stepped up security measures at Moscow airports and railway stations, agencies reported.

The attack followed a statement issued earlier this month by the U.S. Embassy in Moscow that urged Americans to avoid crowded places in the Russian capital in view of an imminent attack, a warning that several other Western embassies repeated.

White House national security advisor John Kirby said Friday that he couldn't yet speak about all the details, but that "the images are just horrible. And just hard to watch."