President Vladimir Putin has said that Moscow would regard a nuclear attack on its allies as a nuclear attack on Russia itself and would immediately respond.

Mr Putin said that Russia had tested an array of new nuclear weapons, including a new nuclear-powered missile at the end of 2017 which could reach almost any point in the world and could not be intercepted by anti-missile systems.

"We would consider any use of nuclear weapons against Russia or its allies to be a nuclear attack on our country. The response would be immediate," Mr Putin said in a speech to Russian MPs.