Russian authorities arrested the four people suspected of taking part in the attack on a suburban Moscow concert hall that killed at least 133 people and believe they were headed to Ukraine, President Vladimir Putin said Saturday during an address to the nation.

Kyiv, meanwhile, strongly denied any involvement in Friday's attack on the Crocus City Hall music venue in Krasnogorsk, which the Islamic State group's affiliate in Afghanistan claimed responsibility for in a statement posted on social media channels linked to the group. Kyiv accused Putin and other Russian politicians of falsely linking Ukraine to the assault in order to stoke fervor in Russia's war in Ukraine, which recently entered its third year.

A US intelligence official told The Associated Press that US agencies had confirmed that IS was responsible for the attack.

Putin said authorities have detained a total of 11 people in the attack, which also injured scores of concertgoers and left the venue a smoldering ruin. He called it “a bloody, barbaric terrorist act” and said Russian authorities captured the four suspected gunmen as they were trying to escape to Ukraine through a “window” prepared for them on the Ukrainian side of the border.

Putin also said that additional security measures have been imposed throughout Russia, and he declared Sunday to be a day of mourning.

Investigators on Saturday were combing through the charred wreckage of the hall for more victims, and the authorities said the death toll could still rise. Hundreds of people stood in line in Moscow early Saturday to donate blood and plasma, Russia's health ministry said.

“We faced not just a thoroughly and cynically prepared terror attack, but a well-prepared and organized mass murder of peaceful innocent people,” Putin said.

The attack, which was the deadliest in Russia in years, came just days after Putin cemented his grip on power in a highly orchestrated electoral landslide and as the war in Ukraine drags on.

Some Russian lawmakers pointed the finger at Ukraine immediately after the attack. But Mykhailo Podolyak, an adviser to Ukrainian President Volodymyr Zelenskyy, denied any involvement.

“Ukraine has never resorted to the use of terrorist methods,” he posted on X. “Everything in this war will be decided only on the battlefield.”

Ukraine's foreign ministry accused Moscow of using the attack to try to stoke fervor for its war efforts.

“We consider such accusations to be a planned provocation by the Kremlin to further fuel anti-Ukrainian hysteria in Russian society, create conditions for increased mobilization of Russian citizens to participate in the criminal aggression against our country and discredit Ukraine in the eyes of the international community,” the ministry said in a statement.

Images shared by Russian state media Saturday showed a fleet of emergency vehicles still gathered outside the ruins of Crocus City Hall, which could hold more than 6,000 people and has hosted many big events, including the 2013 Miss Universe beauty pageant that featured Donald Trump and other VIPs.

Videos posted online showed gunmen in the venue shooting civilians at point-blank range. Russian news reports cited authorities and witnesses as saying the attackers threw explosive devices that started the fire. The roof of the theater, where crowds had gathered for a performance by the Russian rock band Picnic, collapsed early Saturday as firefighters spent hours fighting the blaze.

In a statement posted by its Aamaq news agency, the IS's Afghanistan affiliate said it had attacked a large gathering of “Christians” in Krasnogorsk.

A US intelligence official told the AP that American intelligence agencies had gathered information in recent weeks that the IS branch was planning an attack in Moscow, and that US officials had privately shared the intelligence earlier this month with Russian officials.

The official was briefed on the matter but was not authorized to publicly discuss the intelligence information and spoke to the AP on condition of anonymity.

Messages of outrage, shock and support for the victims and their families have streamed in from around the world.

On Friday, the UN Security Council condemned “the heinous and cowardly terrorist attack” and underlined the need for the perpetrators to be held accountable. U.N. Secretary-General Antonio Guterres also condemned the terrorist attack “in the strongest possible terms,” his spokesman said.

Putin, who extended his grip on Russia for another six years in this week's presidential vote after a sweeping crackdown on dissent, had publicly denounced the Western warnings of a potential terrorist attack as an attempt to intimidate Russians. “All that resembles open blackmail and an attempt to frighten and destabilize our society,” he said earlier this week.

In October 2015, a bomb planted by IS downed a Russian passenger plane over Sinai, killing all 224 people on board, most of them Russian vacation-goers returning from Egypt. The group, which operates mainly in Syria and Iraq but also in Afghanistan and Africa, also has claimed several attacks in Russia's volatile Caucasus and other regions in the past years. It recruited fighters from Russia and other parts of the former Soviet Union.