International Desk

Four men accused of involvement in a massacre at a Moscow concert hall that killed 137 people were remanded in custody Sunday, as Russia observed a national day of mourning following the attack claimed by the Islamic State.

All four suspects have been charged with terrorism, according to Moscow's Basmanny district court, and face life imprisonment. Their detention is set until May 22 but may be extended depending on the date of their trial.

The court said two of the defendants had pleaded guilty, and one of them, from Tajikistan, had "entirely acknowledged his guilt".

President Vladimir Putin has vowed to punish those behind the "barbaric terrorist attack", and on Saturday said the four gunmen had been arrested while trying to flee to Ukraine. Kyiv has strongly denied any connection to the attack.

Putin has made no public reference to the Islamic State (IS) group's claims of responsibility.

At least 137 people, including three children, were killed Friday evening when gunmen stormed the Crocus City Hall in Moscow's northern suburb of Krasnogorsk then set fire to the building.

It is the deadliest attack in Europe to have been claimed by IS.

Russia's Investigative Committee posted a video of the four suspects being dragged into its headquarters in Moscow.

There was no statement on the other seven suspects arrested in connection with the attack.

Officials have said the gunmen were all foreign nationals.

- 'Machine guns, knives, firebombs' -

The Islamic State group posted Saturday on Telegram that the attack was "carried out by four IS fighters armed with machine guns, a pistol, knives and firebombs" as part of "the raging war" with "countries fighting Islam".

A video lasting about a minute and half, apparently filmed by the gunmen, has been posted on social media accounts typically used by IS, according to the SITE intelligence group.

The video -- which appears to have been filmed from the lobby of the concert venue -- shows several individuals with blurred faces and garbled voices, firing assault rifles with inert bodies strewn on the floor and a fire starting in the background.

Russian investigators said that after walking through the theatre shooting spectators, the gunmen set fire to the building, trapping many inside.

Health officials said the number of casualties had risen to 182, with 101 people still in hospital, of whom 40 were in "critical" or "extremely critical" condition.

The attack was the deadliest in Russia since the Beslan school siege in 2004.

The emergency situations ministry has so far named 29 of the victims, but the blaze has complicated the process of identification.

The ministry on Sunday posted a video of heavy equipment arriving at the venue to dismantle damaged structures and clear debris.