Russia observed a national day of mourning on Sunday after a shooting massacre at a concert hall on the outskirts of Moscow.

At least 137 people were killed and more than 140 injured when attackers stormed the Crocus City Hall on Friday.

The Islamic State Khorasan Province (ISIS-K) claimed responsibility for the rampage, but there were indications that Russia was pursuing a Ukrainian link despite firm denials from Kyiv.

It was the deadliest attack in Russia for almost two decades.

Putin says all four gunmen arrested

Russian President Vladimir Putin said all four gunmen responsible for the attack were among the 11 people detained for the attack.

Meanwhile, Moscow's Basmanny District Court formally charged four suspects between the ages of 19 and 32 with committing a deadly group terrorist attack. They face a maximum sentence of life imprisonment. Two of the defendants had pleaded guilty, the court said.

According to Russian news reports, the gunmen were citizens of Tajikistan, a former Soviet republic in Central Asia that is predominantly Muslim and borders Afghanistan.

Putin said they were heading for Ukraine but did not directly implicate Kyiv and vowed to punish those behind the "barbaric terrorist attack."

Ukrainian President Volodymyr Zelenskyy said, "Putin and the other scum are just trying to blame it on someone else," denying any involvement. 

US intelligence officials confirmed the claim by the IS affiliate.

"ISIS bears sole responsibility for this attack. There was no Ukrainian involvement whatsoever," National Security Council spokeswoman Adrienne Watson said.

A video shot by gunmen who carried out the deadly attack on a Moscow concert hall has been posted on social media accounts typically used by the so-called "Islamic State."

Makeshift memorial outside the concert hall

A makeshift memorial was set up outside the concert hall, where people laid flowers for the victims of the attack.

Three children were among those killed, and people also placed toys near the hall, as emergency workers continued to clear the rubble and search for further victims.

In Moscow, long lines formed for people to donate blood. Health officials said more than 120 people were wounded in the attack.

Russia tightened security at airports, transport hubs, and across the capital, and big public events were canceled nationwide.

lo/ss (AFP, AP, dpa, Reuters)