Moscow, Russia - Gunmen opened fire at a rock concert in a Moscow suburb Friday, leaving dead and wounded before a major fire spread through the theatre, Moscow's mayor and Russian news agencies reported.

Attackers dressed in camouflaged outfits entered the building, opened fire, and threw a grenade or incendiary bomb, according to a journalist for the RIA Novosti news agency who was at the scene.

Russia's foreign ministry called the incident a "terrorist attack" that had to be condemned.

The fire quickly spread through the Crocus City Hall, north of the Russian capital, where the theatre can hold several thousand people and has staged several concerts by top international artists, according to the reports.

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No details were given on casualties, but Moscow mayor Sergei Sobyanin confirmed there were deaths in what he called a "terrible tragedy" at the concert by Russian rock band Piknik.

Automatic gunfire was used on the audience, the RIA Novosti journalist reported.

"People who were in the hall were led on the ground to protect themselves from the shooting for 15 or 20 minutes," the journalist was quoted as saying.

People started crawling out when it was safe, the journalist reported, adding that security forces were at the scene.

About 100 people escaped through the theatre basement while others were sheltering on the roof, the emergency services ministry said on its Telegram channel.

Telegram news channels Baza and Mash, which are close to security forces, showed video images of flames and black smoke pouring from the concert hall.

Other images showed two men walking through the hall with at least one person left on the ground near the entrance. Spectators were also seen hiding behind seats or trying to escape.

Russia's foreign ministry spokeswoman Maria Zakharova said it had been a "terrorist attack."

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"The whole international community must condemn this odious crime," she said on Telegram.

The US presidency called the attack "terrible" but said there was no immediate indication of any link to the war in Ukraine.

"I offer my condolences to the families of the dead," said Moscow's mayor as a major security operation was launched around the theatre and nearby shopping mall.

Sobyanin said he had canceled all public events in Moscow for the weekend.

TASS news agency said that SOBR special police forces and the OMON anti-riot squad had been sent to the Crocus hall.

It added that all the members of the rock band had been evacuated safely.

Orthodox church leader Patriarch Kirill was "praying for peace for the souls of the dead," said his spokesman Vladimir Legoyda.

At least 40 people were killed and more than 100 injured in an attack Friday on a concert hall outside Moscow, Russia's Federal Security Service said.

"According to preliminary information, 40 people were killed and more than 100 were injured as a result of a terrorist attack in the Crocus City Hall," the Interfax news agency reported, citing the FSB.

The White House has said that it has no initial indication that Ukraine was involved in the "terrible" attack.

"There is no indication at this time that Ukraine, or Ukrainians, were involved in the shooting," National Security Council spokesman John Kirby told reporters. "I would disabuse you at this early hour of any connection to Ukraine."

The US Embassy had warned on March 7 that "extremists" were planning something with the Russian presidential election just days away, but Kirby said Washington had no prior knowledge of the attack.

Former Russian President Dmitry Medvedev said the country will find and destroy Ukraine's top officials if they are linked to the gun attack.

"If it is established that these are terrorists of the Kyiv regime... all of them must be found and ruthlessly destroyed as terrorists," Medvedev wrote in a post on Telegram, adding that "official representatives of the state that committed such a crime" would also be punished.

Ukrainian presidential aide Mykhailo Podolyak has refuted allegations of the country's involvement in a statement shared on Telegram.

"Regarding the events in Crocus City in the Russian suburbs, where certain events took place today, some shooting, some terrorist actions by unidentified persons, let's be clear, Ukraine absolutely has nothing to do with these events," he said.