Russian President Vladimir Putin said on Saturday that the four men who attacked a concert near Moscow were heading towards Ukraine when they were detained, and that they hoped to cross the border.

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.

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, addressing the nation, said that some people on the Ukrainian side had prepared to let them cross the border from Russia. Ukraine has denied any involvement in the attack.

"They tried to hide and moved towards Ukraine, where, according to preliminary data, a window was prepared for them from the Ukrainian side to cross the state border," Putin said. Putin said 11 people had been detained in total.

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 Sunday would be a day of mourning.

Russia's interior ministry said the gunmen were foreign citizens.

Ukrainian military intelligence spokesman Andriy Yusov told Reuters: "Ukraine was of course not involved in this terror attack. Ukraine is defending its sovereignty from Russian invaders, liberating its own territory and is fighting with the occupiers’ army and military targets, not civilians."

He said the FSB version that the suspects were arrested en route to Ukraine was "of course another lie from the Russian special services". (Reuters)