The Kremlin said on Wednesday that it was not sure of the veracity of a CNN article that reported U.S. President Donald Trump had once threatened to bomb Moscow to deter Russian President Vladimir Putin from attacking Ukraine.

Russian media have been reacting to Trump's growing criticism of Putin, the most recent being on Tuesday when he accused the Russian leader of peddling "bulls***" over his intentions in Ukraine, after rejecting U.S. ceasefire proposals.

The Kremlin has cast doubt on the authenticity of last year's audio recording in which US President Donald Trump recounts how, during his first term, he threatened Russian leader Vladimir Putin that he would "bomb Moscow".