The gunmen, wearing combat fatigues, also caused a huge blaze at the Crocus City Hall. Russian media reported the venue's roof was collapsing.

Some Russian reports suggested people could have been trapped by the fire. Russian media said children were among the dead, citing a Russian MP.

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Islamic State claimed responsibility for the attack on its Telegram channel, Reuters reported.

The news agency later said the US has intelligence confirming the group's claim of responsibility, citing a US official.

"We did warn the Russians appropriately," the official told Reuters.

It was not immediately clear what happened to the attackers after the raid, which state authorities are investigating as terrorism. It was the deadliest attack in Russia in recent years.

Footage taken inside the auditorium shows up to four gunmen firing automatic weapons as people crouch and hide behind seats.

Attackers threw explosives, triggering massive blaze

In other videos posted by Russian media and Telegram channels, extended rounds of gunfire could be heard while men with rifles moved through the venue's mall.

Russian news reports said the attackers threw explosives, triggering the massive blaze at the venue, where flames and plumes of black smoke could be seen rising into the air.

Reports said concertgoers were being evacuated but an unknown number could have been trapped by the blaze.

All tickets in the 6,200-seat concert hall for a performance by Soviet-era rock group Picnic had been sold out.

Analysis: Kyiv worries Putin will exploit the bloodshed in Moscow to escalate his war in Ukraine

'Huge tragedy'

Moscow Mayor Sergei Sobyanin described the attack as a "huge tragedy" and cancelled all mass gatherings scheduled for the weekend.

Russian authorities said security had been tightened at Moscow's airports and railway stations.

President Vladimir Putin was receiving regular updates about the shooting, Kremlin spokesman Dmitry Peskov said.

Two weeks ago the US embassy in Russia warned "extremists have imminent plans to attack large gatherings in Moscow, to include concerts".

It issued the warning several hours after the FSB said it had foiled an attack on a synagogue in Moscow by an Islamic State cell.

'No indication' Ukraine was involved, White House says

White House national security adviser John Kirby said he could not yet speak about all the details but that "the images are just horrible. And just hard to watch".

Mr Kirby said there was "no indication at this time that Ukraine, Ukrainians were involved".

But Russia said if the US knew for sure Ukraine was not involved then it should share any information it had.

Russian foreign ministry spokeswoman Maria Zakharova said: "On what basis do officials in Washington draw any conclusions in the midst of a tragedy about someone's innocence?"

She said if Washington had information it should be shared and if it had no information it should not be talking in such a way.

Ukraine denies involvement

Ukraine's presidential adviser Mykhailo Podolyak said Kyiv had nothing to do with the attack.

"Let's be straight about this: Ukraine had absolutely nothing to do with these events," Mr Podolyak said.

"We have a full-scale, all-out war with the Russian regular army and with the Russian Federation as a country. And
regardless of everything, everything will be decided on the battlefield."

It comes days after Mr Putin secured his fifth term as Russia's president in an electoral landslide.

Russia was shaken by a series of deadly terror attacks in the early 2000s as it fought separatists in the Russian province of Chechnya.

In October 2002 Chechen militants took around 800 people hostage at a theatre in Moscow which was stormed by Russian special forces two days later, leaving 129 hostages and 41 Chechen fighters dead, most from the effects of the gas Russian forces used to subdue the attackers.

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