Russian authorities have charged four suspects with terrorism over the deadly attack on a concert hall near Moscow.

The Russian Investigative Committee says 137 people died in Friday's attack in the city of Krasnogorsk. Health officials say 182 are undergoing treatment at hospitals.

A court in Moscow said on social media on Sunday that two of the suspects have admitted to the charges against them, and identified one of them as a Tajik national.

The Amaq news agency, which is linked to Islamic State militants, says the attack was carried out by the group's members. It posted an online video showing men firing automatic weapons.

Russian President Vladimir Putin has hinted that Ukraine was involved in the attack. Ukrainian President Volodymyr Zelenskyy denied that his country was involved.

Russia observed a day of mourning on Sunday. Flags were flown at half-mast across the country. Many people visited the area around the attack site and left flowers to pay tribute to the victims.