House Speaker Paul Ryan said he will focus on the election of his party's congressional candidates

Paul Ryan, the top Republican in the US Congress, took the extraordinary step on Monday of distancing himself from Donald Trump, stirring a backlash from some lawmakers and deepening a crisis over his party's struggling presidential nominee.

In a conference call with congressional Republicans, Ryan all but conceded that Democrat Hillary Clinton was likely to win the White House on Nov 8 and said he would put his full energy into preserving Republican majorities in Congress so as not to give her a "blank check."

Ryan, the Speaker of the House of Representatives, said he would not defend Trump or campaign for him after the uproar over the New York businessman's sexually aggressive comments that surfaced on Friday.

Ryan's announcement added to the party's worst turmoil in decades and reinforced the growing sense of isolation around Trump, who has never previously run for public office.

Clinton has led Trump in most national opinion polls for months and Trump's poll numbers have begun to drop further since the emergence on Friday of a video from 2005 showing the former reality TV star bragging crudely about groping women and making unwanted sexual advances.

Trump hit back at Ryan, the Republican vice-presidential candidate in 2012, who has frequently been critical of him.

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

Ryan, who had expressed disgust over the tape and canceled a campaign event with Trump over the weekend, did not completely cut ties to Trump. The speaker went back on the Republican conference call later to clarify he was not withdrawing his endorsement.

Many Republican members of Congress are concerned that Trump's chaotic campaign could ruin their chances of holding their majorities in the House of Representatives and Senate in the November election and could inflict long-term damage on the party.

During a weekend dominated by criticism of Trump over the lewd remarks, a string of members of Congress, governors and other prominent Republicans called on him to drop out of the race.

House Republicans gave Ryan a rough ride on the call, according to some participants.

"There was an undeniable opposition to the speaker's tepid support of our nominee," said US Representative Scott DesJarlais, a Trump supporter, in a comment passed on by an aide.

Many other lawmakers, some of whom did not want to be named publicly criticizing the speaker, said members frequently told Ryan on the call to stand by Trump.

Nonetheless, nearly half of all 331 incumbent Republican senators, Congress members and governors have condemned Trump's remarks, and roughly one in 10 has called on him to drop out of the race, according to a Reuters review of official statements and local news coverage.

Republican National Committee Chairman Reince Priebus used an afternoon conference call with RNC members to emphasize there was no rift with Trump and that the committee, the party's leadership and fundraising arm, still backed the nominee, two RNC members who spoke on condition of anonymity said.

"Any suggestion that the RNC isn't fully supporting the Trump-Pence ticket is wrong," one RNC member said, describing the message. "We are fully on board. We are going to devote every ounce of effort and resource into helping the Trump-Pence ticket win and all the other candidates up and down the ballot."

House Speaker Paul Ryan gestures to reporters on Capitol Hill in Washington last week. Cliff Owen / Associated Press