US President Donald Trump on Monday dubbed all sexual assault allegations against him "fake news" after his campaign was subpoenaed as part of a defamation suit brought by a former reality show contestant.

"All I can say is it's totally fake news. It's fake and made-up stuff and it's disgraceful what happens, but that happens in the world of politics," he told reporters at the White House.

The lawsuit by Summer Zervos, a former contestant on Trump's TV show "The Apprentice," says he made "numerous false, defamatory statements" in response to her allegations that he tried to kiss and grope her without her consent.