Moscow, Russia – Russian investigators said Saturday the death toll from a shooting and fire at a Moscow concert hall had risen to 133 from an earlier figure, as President Vladimir Putin vowed revenge.

"While clearing the debris in the Crocus City Hall concert hall, the number killed as a result of the terrorist attack rose to 133 people. Search operations are continuing," the Investigative Committee said in a statement on Friday's attack.

The Islamic State (IS) jihadist group said Saturday four of its militants carried out an attack on a concert hall in a Moscow suburb that Russian authorities said killed at least 133 people. Russian investigators on Saturday said the "terrorists" then set fire to a Moscow concert hall using a "flammable liquid"

"The attack was carried out by four IS fighters armed with machine guns, a pistol, knives and firebombs," IS said on one of its Telegram channels, adding that the attack was part of "the natural context of the raging war" with "countries fighting Islam."

Russian President Vladimir Putin on Saturday called the attack a "barbaric terrorist act" and vowed harsh retribution to all those involved. In a televised address, he said all four gunmen had been arrested before they had a chance to cross the border into Ukraine.

"I am speaking to you today in connection with the bloody, barbaric terrorist act, the victims of which were dozens of innocent, peaceful people," Putin said in his first public remarks since the attack.

"All four perpetrators of the terrorist attack who shot and killed people have been detained. They were travelling towards Ukraine where, according to preliminary information, they had a window to cross the border," the Kremlin leader said.

Russia's FSB security service said earlier the assailants had been "in contact" with people in Ukraine as they tried to flee the country. The country confirmed it has arrested 11 people – including four gunmen – over Friday's attack.

"Terrorists, murderers, non-humans will face the unenviable fate of retribution and oblivion," said Putin, who declared Sunday a day of national mourning across Russia.

UN human rights chief Volker Turk was "horrified" by the deadly Moscow concert hall attack, his office said Saturday, as it called for the perpetrators to be held to account.

"Nothing justifies such an attack. Perpetrators must be held accountable in line with human rights law. Our solidarity with the victims of this shocking violence," the UN Human Rights Office said on X, formerly Twitter.

Camouflaged gunmen opened fire at the packed hall in Moscow's northern suburb of Krasnogorsk ahead of a concert by rock band Piknik in the deadliest attack in Russia for at least a decade.

UN Secretary General Antonio Guterres and the UN Security Council earlier made the "strongest" condemnations of the attack and offered condolences to the victims of the "terrorist" incident.

Rescuers will for "several days" scour through the rubble of a Moscow concert hall where gunmen shot dead dozens of people, the Moscow region's governor said Saturday.

"Rescuers are working at the site of the tragedy round the clock ... the work will continue for at least several more days," said Andrey Vorobyov in a post on Telegram.