At least 133 people have been killed and more than 100 injured when four gunmen on Friday night dressed in camouflage opened fire with automatic weapons at people at a concert in the Crocus City Hall near Moscow, in one of the worst such attacks on Russia in years claimed by the ISIL (Daesh) group.

Flames leapt into the sky and plumes of black smoke rose above the venue, Reuters pictures and video showed.

Russian media reported a second blast at the venue and there were reports that some of the gunmen had barricaded themselves somewhere in the building.

The shooting appeared to have begun at a concert of the band "Picnic".

"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 investigators said about 60 of the injured were in critical condition.

"A terrible tragedy occurred in the shopping center Crocus City today," Moscow Mayor Sergei Sobyanin said. "I am sorry for the loved ones of the victims."

Russia tightened security at airports and stations and across the capital - a vast urban area of over 21 million people. 

Russia's foreign ministry said it was a "bloody terrorist attack".

"The entire world community is obliged to condemn this monstrous crime," Foreign Ministry Spokeswoman Maria Zakharova said. "All efforts are being thrown at saving people."

ISIL, the terrorist group that once sought control over Iraq and Syria, claimed responsibility for the attack on its Telegram channel, saying the gunmen had escaped. 

On Saturday, a spokesperson for Russia’s Investigative Committee said it was too early to say anything about the fate of the attackers, state news agency RIA reported.

Shooting people

In one unverified video posted on social media, men with automatic weapons were shown firing repeatedly at screaming civilians, including women, who were cowering below what looked like an entrance sign to "Crocus City Hall".

Other video footage showed a number of people lying motionless in pools of blood outside the hall. Reuters was unable to immediately verify the footage.

Another video showed the attackers shooting at people in the concert hall.

The White House said on Friday that images of shooting in the Russian capital were hard to watch.

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

The U.S. embassy in Russia warned earlier this month that "extremists" had imminent plans for an attack in Moscow.

It issued its warning several hours after the FSB, the main successor to the Soviet-era KGB, said it had foiled an attack on a synagogue in Moscow by a cell of the militant Sunni Muslim group Islamic State.