United States House Speaker Paul Ryan on Monday told Republican lawmakers on a conference call that he will no longer "defend" or campaign with presidential nominee Donald Trump, focusing instead on maintaining his party's majority in Congress.

Ryan, who has had a testy relationship with the provocative billionaire from the start and has criticised him on numerous occasions, stopped short of rescinding his endorsement, according to a person who listened to the conference call.

Ryan "said he will not defend Trump or campaign with him for the next 30 days," the source on the call said.

But a spokeswoman for Ryan, AshLee Strong, told reporters after the call that there was "no update in his position at this time" in terms of endorsing Trump.

The speaker will "spend the next month focused entirely on protecting our congressional majorities," Strong said in an email.

But the source on the call appeared to suggest Ryan had effectively conceded the election to Trump's Democratic rival Hillary Clinton, who is leading in national polls and in several key battleground states.

Hitting back at Ryan, Trump said that the House of Representatives' Speaker should "not waste his time" opposing the party's nominee.

"Paul Ryan should spend more time on balancing the budget, jobs and illegal immigration and not waste his time on fighting Republican nominee," Trump said in a post on Twitter.

Trump's candidacy suffered a crippling blow after a 2005 tape was released Friday in which he made lewd and derogatory comments about women, including his ability to grab them by the crotch with impunity because, as a celebrity, "you can do anything."

In a bitter televised debate, a month before the US presidential election, Trump denied he had groped anyone.