Leaders around the world expressed their condolences and condemned the terror attack.

United Nations Secretary-General Antonio Guterres condemned the attack, saying on Saturday at a news conference at El Arish International Airport in Egypt that ISIS is a significant global threat.

Putin linked the attack, which he called "barbaric", to Ukraine in a video statement released on Saturday, as he expressed deep condolences and declared Sunday a national day of mourning.

On Ukrainian national TV on Saturday, Andrii Yusov, spokesman for Ukraine's Defense Intelligence, firmly denied that his country had anything to do with the terror attack.

United States Secretary of State Antony Blinken denounced the attack as a "heinous crime" and said Washington condemns "terrorism in all its forms".

"We ... stand in solidarity with the people of Russia in grieving the loss of life from this horrific event," Blinken said in a statement on Saturday.

Earlier, the White House had called the attack "terrible" and offered condolences.

European Commission President Ursula von der Leyen said in a post on social media platform X on Saturday that she "strongly condemns" Friday's concert hall attack. "My thoughts are with the victims and their families during this tragic time," she said.

Turkish President Recep Tayyip Erdogan sent his condolences to the Russian government and denounced the attack as well. "Regardless of the origin of the suspects, terror cannot be accepted," he said.

Israeli President Isaac Herzog said he spoke with Russia's ambassador to Israel "to convey — on behalf of the Israeli people — my condolences to the families of the victims, to the Russian people and its leadership for the terrible loss of life", adding that he "wished a speedy recovery to all those injured".

Other European leaders, including those of France, Poland, the United Kingdom and Germany, also denounced the attack.

Agencies contributed to this story.