US intelligence agencies accused Vladimir Putin of launching an "influence campaign" to damage Hillary Clinton in a new report, with sources saying British intelligence provided the tip about Russia's hacking of the Democratic Party.

The report said Russia showed a "clear preference" for Donald Trump, the president-elect, and carried out cyber attacks and issued propaganda both to boost his chances and to undermine confidence in American democracy.

Mr Trump insisted on Friday that foreign meddling had "absolutely no effect" on the outcome of the election, and declined to say whether he believed Russia was behind the hacks.