Editor's note: Russia observed a national day of mourning on Sunday after the Moscow concert hall massacre on Friday, which President Vladimir Putin called a "barbaric terrorist attack". China Daily looks at the solidarity shown toward the attack victims and the calls made for concerted global efforts to counter terrorism.

The deadly attack on a concert hall outside Moscow on Friday has raised serious concerns about security in Russia, as experts call for increased international collaboration to combat terrorism worldwide.

With the death toll reaching 137, including three children, and more than 150 individuals injured, Russia's President Vladimir Putin declared a national day of mourning on Sunday.

It was the deadliest attack on Russian territory since the Beslan school siege nearly 20 years ago, when Islamist militants took more than 1,000 people, including hundreds of children, hostage. That siege resulted in the deaths of 334 people.

In a televised address, Putin, who called the attack "a bloody, barbaric terrorist act", cast the enemy as "international terrorism" and said he was ready to work with any state that wanted to defeat it.

The Kremlin said Putin had held conversations with the leaders of Belarus, Uzbekistan and Kazakhstan in which all sides affirmed their willingness to work together to fight terrorism, Reuters reported.

Following the attack, Russia bolstered security measures at airports, transportation hubs and throughout Moscow, and large public events across the country were canceled as a precautionary measure.

Wan Qingsong, an associate research professor at the Russian Studies Center of East China Normal University in Shanghai, said Russia now faces significant pressure in tackling extremist threats and maintaining security within its borders.

"Russia has assumed a pivotal role in the global fight against terrorism, exerting significant influence that cannot be ignored. However, it has been grappling with security vulnerabilities, which have been exploited by terrorist organizations, resulting in numerous attacks."

Alexander Bastrykin, head of Russia's Investigative Committee, had reported that in 2022 a total of 4,729 offenses involving foreign nationals were reported, leading to the opening of 4,231 criminal cases, Wan said. This figure represents a threefold increase compared with the numbers reported in 2021.

"It is evident that behind these recent assaults, there are foreign influences at play, and terrorist entities and operatives have proliferated rapidly, particularly in smaller cities where security measures are weaker," Wan said.