The director of Russia's most powerful security agency said on Tuesday that he believed Ukraine, along with the United States and Britain, were involved in the attack on a concert hall just outside Moscow that killed at least 139 people.

Ukraine, which has repeatedly denied any link with Friday's attack, dismissed the Russian accusations as lies. Britain said they were "utter nonsense." The Islamic State group, a militant organization that once sought control over swathes of Iraq and Syria, claimed responsibility for the mass shooting.

"We believe that the action was prepared by both the Islamist radicals themselves and was facilitated by Western special services," Alexander Bortnikov, director of the Federal Security Service (FSB), said on television.