A Moscow court has ordered the detention of four suspects in the largest terrorist attack in Russia in nearly two decades, as the country held a national day of mourning on Sunday for victims of the attack, which killed at least 137 people and injured 180.

The suspects, who are accused of shooting concertgoers at a 6,200-seat music hall in suburban Moscow on Friday evening and setting the building on fire, appeared before Basmanny District Court late on Sunday. Their identities were confirmed for the first time, and they were placed in custody pending trial.

The four were charged on Sunday at the court with acts of terrorism in connection with the attack.

All the suspects were identified as citizens of Tajikistan, the Tass state news agency said. Several of the men asked for translators, as they could not speak Russian.

Russian President Vladimir Putin spoke with Tajikistan's President Emomali Rahmon in a phone call, in which Rahmon condemned the attack and said the two countries would continue working closely together to fight terrorism and extremism, the Tajik government's press service said on Sunday.

The Russian Investigative Committee said it had found two Kalashnikov rifles, 28 magazines and more than 500 rounds of ammunition at the scene of the attack.

On Sunday, the suspects were led into the headquarters of the committee in Moscow for questioning, their hands tied and their eyes covered with bandages and scarves.

They were then transported to the Basmanny court, where they were accused of terrorism and placed in pretrial detention until May 22 as the investigation into the attack is conducted.

The suspects were brought before the court and media individually, each sitting in a glass box, and three of them pleaded guilty to all charges, Tass said.

The Islamic State terror group has claimed responsibility for the attack. It shared a photograph taken before the attack of the four alleged assailants with a black ISIS flag, as well as a graphic video made by one of the attackers during the rampage.

Friday's attack was the deadliest on Russian territory since the 2004 Beslan school siege, when Islamist militants took more than 1,000 hostages. Over 300 people died, more than half of them children.

Dmitry Medvedev, deputy chairman of Russia's Security Council, said Russia would target those behind the deadly shooting wherever they are from and whoever they are.

Medvedev had previously spoken of the need to meet "death with death", and some lawmakers have begun to discuss whether the death penalty should be reintroduced.

Russia's Federal Security Service said the gunmen had contacts in Ukraine and were captured near the border.

The claim was dismissed by Ukrainian President Volodymyr Zelensky. Ukrainian Foreign Minister Dmytro Kuleba said in a statement on the social media platform X on Sunday that "there is no evidence to support such claims".

The White House said that Washington had shared information with Moscow early this month about a planned attack, and issued a public advisory to United States citizens in Russia on March 7. It said Islamic State bore sole responsibility for the attack.

"Nothing was passed," Russian Ambassador to the US Anatoly Antonov responded early on Sunday, saying that Washington did not share any specific information with the embassy before the terrorist attack.