A few men in camouflage entered into a large concert hall in Moscow and opened fire, killing over 60 people, injuring about 145 and setting the venue on fire on Friday.

The attack is being referred to as the deadliest in years, according to a report in the Associated Press, which Moscow Mayor Sergei Sobyanin called a "huge tragedy," occurring just days after Russian President Vladimir Putin resumed power by securing a landslide victory in the elections.

Soon after the attack, the Islamic State group claimed responsibility for the attack in a statement over social media.

According to some Russian media outlets, the assailants fled before special forces and riot police arrived.

US intelligence officials confirmed the claim by the Islamic State group's branch based in Afghanistan that it was responsible for the Moscow attack, a US official told the AP.

According to the official, the US intelligence agencies had gathered information in the past weeks that the IS branch was planning an attack in Moscow, which was shared with the Russian officials earlier this month. However, the officials were not authorized to publicly discuss the information.

Security was tightened at Moscow's airports, railway stations and the capital's sprawling subway system, according to Russian officials. Moscow's mayor too canceled all mass gatherings, whereas theatres and museums were shut for the weekend. Other Russian regions also tightened security.

According to the Associated Press, on March 7, Russia's top security agency said it thwarted an attack on a synagogue in Moscow by an Islamic State cell, killing several of its members in the Kaluga region near the Russian capital. A few days earlier, Russian authorities said six alleged IS members were killed in a shootout in Ingushetia in Russia's volatile Caucasus region.