Britain, the United States and Ukraine have been blamed for the Moscow concert hall onslaught which killed at least 139, according to the director of Russia's Federal Security Service (FSB).

Alexander Bortnikov now claims the UK is responsible for the terror attack, despite Moscow initially holding Ukraine accountable, Russian state news outlet TASS reports.

Ukraine has denied Russian accusations they were involved in the attack - which the Islamic State militant group claimed responsibility for.

The US has said its intelligence indicates ISIS-K - an Afghan branch of IS - was responsible.