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 camouflage uniforms 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.

"There are dead and injured following the attack at the Crocus City Hall," the FSB security service said. Russia's foreign ministry called the incident a "terrorist attack" and authorities said an investigation is under way.

Fire quickly spread through the 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.

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.

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. He cancelled all public events in Moscow 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, adding that security forces were at the scene.

  'Odious crime'  

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

But about one third of the complex was ablaze, TASS news agency 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 war 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. "I would disabuse you at this early hour of any connection to 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.

TASS said that SOBR and 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.

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.

Russia launched a military intervention in Ukraine in February 2022 and it has been the target of attacks along the border by anti-Kremlin forces.