Russian President Vladimir Putin has accused Ukraine of being behind the deadly attack on the Crocus City Hall concert venue in Moscow on Friday night, which claimed the lives of at least 133 people. In a public address, Putin called the attack a "barbaric act of terrorism" and vowed to punish the perpetrators, as reported by NBC News.

According to Russian security forces, all four direct participants in the attack have been apprehended while attempting to flee to Ukraine. Putin stated, "They tried to hide and moved towards Ukraine, where, according to preliminary data, a window was prepared for them on the Ukrainian side to cross the state border." A total of 11 people have been detained so far in connection with the attack.

However, Kyiv has strongly denied any involvement in the assault. Ukrainian presidential advisor Mykhailo Podolyak called attempts to link the two "absolutely untenable," stating, "Ukraine has not the slightest connection to this incident. Ukraine has a full-scale war with Russia and will solve the problem of Russia's aggression on the battlefield."

The Islamic State militant organization's news agency, Amaq, has claimed responsibility for the attack, posting a picture of the four alleged attackers. Citing security sources, Amaq stated that the attack resulted in "at least 300 Christians killed or wounded" and occurred within the "normal context of the raging war between the Islamic State and countries fighting Islam." The group also claimed that the attack was carried out by four IS "fighters" armed with machine guns, a pistol, knives, and incendiary bombs, preceded by "an intensive monitoring operation."

U.S. intelligence officials have confirmed the claim by the IS affiliate, revealing that they had gathered information in recent weeks about the group's plans to attack Moscow. This intelligence was privately shared with Russian officials earlier this month, according to a U.S. official briefed on the matter.

The death toll from the shooting and fire attack has risen to 133, making it one of the worst acts of terrorism in contemporary Russian history, surpassing the 2002 hostage crisis at Moscow's Dubrovka theater, which claimed 130 lives. In response to the attack, Russia has increased security in its capital city at transportation hubs and airports, and significant public events have been canceled nationwide.

Putin declared Sunday a day of mourning and expressed his condolences to those who lost loved ones in the attack. "The whole country, our entire nation, mourns with you," he said.

The assault has been a major embarrassment for the Russian leader, coming just days after he cemented his grip on the country for another six years in a vote that followed the harshest crackdown on dissent since Soviet times. Some commentators on Russian social media have questioned how authorities failed to prevent the attack despite U.S. warnings.

Ukraine's foreign ministry accused Moscow of using the attack to build support for its war efforts, calling the accusations 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 international community has expressed outrage, shock, and support for the victims and their families. The U.N. Security Council condemned the attack and underlined the need for the perpetrators to be held accountable, while U.N. Secretary-General Antonio Guterres condemned the terrorist attack "in the strongest possible terms."