The attackers, wearing combat fatigues, also threw explosives causing a huge blaze at the 6,200-seat Crocus City Hall in the city of Krasnogorsk, 15 miles to the west of the capital.

At least 145 people were also injured in the assault on Friday evening, shortly before Soviet-era rock group Picnic was to due to perform.

Follow latest: Islamic State says it carried out gun attack

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

Video also showed concert-goers fleeing for the exits as the sound of shooting echoed above screams.

Russian investigators have published pictures of a Kalashnikov automatic weapon, ammunition vests with multiple spare magazines and bags of spent bullet casings.

In the aftermath, flames leapt into the sky, and plumes of black smoke rose above the venue as hundreds of blue lights
from emergency vehicles flashed in the night.

Helicopters sought to douse the fire that engulfed the building and caused parts of the roof to collapse.

It was the deadliest attack in the country since the 2004 Beslan school siege when Islamist militants took more than 1,000 people, including hundreds of children, hostage.

A US official said Washington had warned Moscow in recent weeks of the possibility of an attack.

National Security Council spokesman Adrienne Watson said: "Earlier this month, the US government had information about a planned terrorist attack in Moscow - potentially targeting large gatherings, to include concerts - which prompted the State Department to issue a public advisory to Americans in Russia."

"The US government also shared this information with Russian authorities in accordance with its longstanding 'duty to warn' policy."

The attack comes days after Russian President Vladimir Putin told the board of the Federal Security Service that "Western structures" had been engaging in "outright blackmail" when they warned Moscow of potential terrorist attacks.

He has repeatedly warned foreign powers were seeking to sow chaos inside Russia.

While US intelligence has confirmed Islamic State's claim of responsibility for the massacre, Russia has yet to say who it thinks is responsible.

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

Foreign ministry spokeswoman Maria Zakharova said it was a "bloody terrorist attack" that the entire world should
condemn.

The US was among many countries to express condolences, while Ukraine has denied any involvement.

The United Nations Security Council has condemned what it called a "heinous and cowardly terrorist attack".

Mr Putin changed the course of the Syrian civil war by intervening in 2015, supporting President Bashar al Assad
against the opposition and IS.

In recent weeks the FSB said it had foiled an attack on a Moscow synagogue by an IS affiliate in Afghanistan, known ISIS-K, which has frequently criticised the Russian leader in its propaganda

It was not immediately clear what happened to the militants after the raid on Friday.

IS claimed its insurgents had attacked the venue "killing and wounding hundreds and causing great destruction to the place before they withdrew to their bases safely".

In response, Russia tightened security at airports and transport hubs across the capital, while all major public events were cancelled nationwide.

Mr Putin, who was re-elected for a new six-year term on Sunday, is being regularly updated, according to the Kremlin.

Moscow mayor Sergei Sobyanin said: "A terrible tragedy occurred in the shopping centre Crocus City today.

"I am sorry for the loved ones of the victims."