A new book alleges that President Donald Trump told donors he threatened Russian President Vladimir Putin with bombing Moscow if Russia invaded 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 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.