President Donald Trump issued a rare threat of nuclear escalation on Friday, saying he had ordered two nuclear submarines to be strategically positioned near Russia in response to what he said were aggressive remarks by Dmitry Medvedev,

Russia urged caution after Trump claimed he redeployed nuclear subs in response to provocative remarks by a senior Russian official.

Russia has abandoned a long-held nuclear "freeze" after Donald Trump deployed submarines near Vladimir Putin's coasts. Moscow has said it no longer considers itself bound by a self-imposed ban on the deployment of missiles - and is poised to bring short- and intermediate-range nuclear weapons into action.