Gunmen opened fire at a rock concert in a Moscow suburb Friday, killing at least 40 people, wounding 100, and setting off an inferno in the theatre, authorities said.

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

Fire quickly spread through the Crocus City concert hall in the Krasnogorsk suburb in the north of the Russian capital, which can hold several thousand people and has hosted top international artists.

“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,” according to the FSB security service, quoted by Interfax news agency and other Russian media.

Authorities said a “terrorist” investigation had been started, and President Vladimir Putin was receiving “constant” updates on the attack, his spokesman Dmitry Peskov told Russian news agencies.

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

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

Moscow mayor Sergei Sobyanin said there had been a “terrible tragedy” at the concert by Russian rock band Piknik and cancelled all public events in the city for the weekend.

Security services quoted by Interfax news agency said between two and five people “wearing tactical uniforms and carrying automatic weapons” opened fire on guards at the entrance and then started shooting at the audience.

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

People started crawling out when it was safe, the journalist reported.

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

But about one-third of the complex was ablaze, TASS reported.

Russia’s foreign ministry spokeswoman Maria Zakharova said it had been a “bloody terrorist attack”.

“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 sign of any link to the conflict in Ukraine.

“There is no indication at this time that Ukraine, or Ukrainians were involved in the shooting,” US National Security Council spokesman John Kirby told reporters in Washington.

Former Russian president Dmitry Medvedev vowed on Telegram that Ukraine’s top officials “must be found and ruthlessly destroyed as terrorists” if they were linked to the attack.

“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.

TASS 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.

Moscow and other Russian cities have been the targets of previous attacks by Islamist groups, but there have also been incidents without any clear political motive.

Earlier this month, the US embassy in Russia warned that “extremists have imminent plans to target large gatherings in Moscow”, including concerts.

In 2002, Chechen separatist fighters took 912 people hostage in a Moscow theatre, the Dobrovka, demanding the withdrawal of Russian troops from the region.

Special forces attacked the theatre to end the hostage-taking, and 130 people were killed, nearly all suffocated by a gas used by security forces to knock out the gunmen.

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