Putin, addressing the nation, said that some people on the Ukrainian side had prepared to let them cross the border from Russia.

In a televised address, Putin said 11 people had been detained, including the four gunmen. “They tried to hide and moved towards Ukraine, where, according to preliminary data, a window was prepared for them on the Ukrainian side to cross the state border,” he said.

Militant Islamist group Islamic State claimed responsibility for Friday’s rampage but there were indications that Russia was pursuing a Ukrainian link, despite emphatic denials from Ukrainian officials that Kyiv had anything to do with it.

The FSB security service said the gunmen had contacts in Ukraine and were captured near the border. It said they were being transferred to Moscow.

Neither Putin nor the FSB publicly presented any proof of a link with Ukraine, with which Russia has been waging war for the past 25 months.

Russia’s state Investigative Committee said 133 people had been killed. State TV editor Margarita Simonyan, without citing a source, had earlier given a toll of 143.

Putin cast the enemy as “international terrorism” and said that he was ready to work with any state which wanted to defeat it.

“All the perpetrators, organisers and those who ordered this crime will be justly and inevitably punished. Whoever they are, whoever is guiding them,” Putin said.

“We will identify and punish everyone who stands behind the terrorists, who prepared this atrocity, this strike against Russia, against our people.”

He said March 24 would be a day of mourning.