Vladimir Putin has moved at least four heavy Tu-95MS nuclear bombers closer to Europe, says a report.

The strategic aircraft are often used to bomb Ukraine, and the report from Insider UA says they have been loaded with missiles, leading to a 'high probability' of an imminent major attack.

But, if confirmed, the move comes after Donald Trump ordered two nuclear submarines 'closer to Russia' after a threat of atomic war from the Kremlin dictator's close ally, Dmitry Medvedev.

The US submarines are now 'where they have to be', Trump said late on Sunday.

Putin rebased his Tu-95MS planes from Olenya air base in Arctic Murmansk region after an audacious June 1 drone strike by Ukraine.