Moscow. Several gunmen burst into a large concert hall on the edge of Moscow on Friday and sprayed visitors with automatic gunfire, killing at least 40 and injuring 100 others in an attack that came just days after President Vladimir Putin cemented his grip on the country in a highly orchestrated electoral landslide.

There were no immediate claims of responsibility for the raid, which Moscow Mayor Sergei Sobyanin described as a “huge tragedy" and which state authorities were investigating as an act of terrorism. It was the deadliest attack in Russia in years and came as the country's war in Ukraine dragged into a third year.

Russia’s Federal Security Service, the main domestic security and counter-terrorism agency, said 40 people were killed and more than 100 were wounded in the attack at Crocus City Hall, a large music venue on Moscow’s western edge.

Russian news reports said that the assailants threw explosives, triggering the massive blaze at the hall, which can accommodate 6,000. Video from outside showed the building on fire, with a huge cloud of smoke rising through the night sky. The street was lit up by the blinking blue lights of dozens of firetrucks, ambulances and other emergency vehicles.

The attack took place as crowds gathered for a performance by Picnic, a famous Russian rock band. Russian news reports said concertgoers were being evacuated, but that an unknown number could have been trapped by the blaze.

The prosecutor’s office said several men in combat fatigues entered the concert hall and fired on concertgoers.

Extended rounds of gunfire could be heard in videos posted by Russian media and on Telegram channels. One showed two men with rifles moving through the venue. Another showed a man inside the auditorium and saying the assailants had set it on fire, as gunshots rang out incessantly in the background.

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

Andrei Vorobyov, the governor of the Moscow region, said he was heading to the area and set up a task force to deal with the damage. He didn’t offer further details.

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

Russian authorities said security was tightened at Moscow’s airports and railway stations, while the Moscow mayor canceled all mass gatherings scheduled for the weekend.

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.”

“Our thoughts are going to be with the victims of this terrible, terrible shooting attack,” Kirby said. “There are some moms and dads and brothers and sisters and sons and daughters who haven’t gotten the news yet. This is going to be a tough day.”

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

Putin, who extended his grip on Russia for another six years in the March 15-17 presidential vote after a sweeping crackdown on dissent, earlier this week denounced the Western warnings as an attempt to intimidate Russians.

Russia was shaken by a series of deadly terror attacks in the early 2000s during the fighting with separatists in the Russian province of Chechnya.

In October 2002, Chechen militants took about 800 people hostage at a Moscow theater. Two days later, Russian special forces stormed the building and 129 hostages and 41 Chechen fighters died, most of them from the effects of narcotic gas Russian forces used to subdue the attackers.

In September 2004, about 30 Chechen militants seized a school in Beslan in southern Russia taking hundreds of hostages. The siege ended in a bloodbath two days later and more than 330 people, about half of them children, were killed.