KRASNOGORSK, Russia (AFP) — Gunmen who opened fire at a Moscow concert hall killed more than 60 people and wounded over 100 while sparking an inferno, authorities said Saturday, with the Islamic State group claiming responsibility.

Attackers dressed in camouflage uniforms entered the building on Friday, 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 Moscow’s northern Krasnogorsk suburb, as smoke filled the building and screaming visitors rushed to emergency exits.

Alexei, a music producer, was about to settle into his seat before the start of a concert by Soviet-era rock band Piknik when he heard gunfire and “a lot of screams”.

“I realised right away that it was automatic gunfire and understood that most likely it’s the worst — a terrorist attack,” said Alexei, who would not give his last name.

As people ran towards emergency exits, “there was a terrible crush” with concert-goers climbing on one another’s heads to get out, he added.

Russia’s Health Minister Mikhail Murashko said 115 people were hospitalised, including five children, one of whom was in grave condition. Of the 110 adult patients, 60 were in serious condition.

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

The Islamic State group said its fighters attacked “a large gathering” on Moscow’s outskirts and “retreated to their bases safely”.