President Vladimir Putin has drawn a "red line" for the United States and its allies by signalling that Moscow will consider responding with nuclear weapons if they allow Ukraine to strike deep inside Russia with long-range Western missiles.

Dmitry Medvedev, a long-time ally of Russian President Vladimir Putin, has laid out the country's new deterrence policy against Western "aggression," and warned that provocations by Ukraine and its allies could result in nuclear war.

The Russian president said an attack on Russia using conventional weapons supported by a nuclear state may trigger a nuclear response.