The chairman of America's House of Representatives' intelligence committee is facing mounting pressure to recuse himself from its Russia probe after he admitted meeting the source behind his surveillance claims on White House grounds.

Devin Nunes sparked controversy last week when he revealed that Donald Trump had been the subject of "incidental surveillance" by US intelligence.

He said a source had come forward to show him intelligence reports which included Mr Trump's communications, offering the president ammunition for previous claims that he had been spied on by the Obama administration.

The reports showed spy agencies may have inadvertently captured Mr Trump and his associates during routine targeting of foreigners' communications. Details of US citizens are supposed to be "masked" in such reports, but Mr Trump and others had been "unmasked".