Media playback is unsupported on your device Media caption Trump v Clinton: Sex, lies and videotape

The most senior elected US Republican official has said he will not defend Donald Trump, after remarks he made about groping women led to outrage.

House of Representatives Speaker Paul Ryan vowed to focus on defending seats in Congress, but did not end his endorsement of the party's nominee.

Mr Trump tweeted that Mr Ryan should not waste his time fighting him.

Earlier Democratic rival Hillary Clinton cast doubt on Mr Trump's apology for the 11-year-old remarks.

On Sunday, Mr Trump described his words as "locker-room talk".

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

Mrs Clinton tweeted on Monday that, if he stood by this assertion, he was "clearly not sorry".

A 2005 video released on Friday revealed Mr Trump describing how he had sought to have sex with a married woman and making obscene comments about women.

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Walking a fine line, by Anthony Zurcher, BBC News

Paul Ryan, the highest-ranking Republican officeholder, has officially given the signal. The SS Trump is sinking, and it's time for members of his party to calmly, quietly head to the lifeboats.

Republican control of Congress must be maintained at all costs, the House speaker asserted in his call to congressional rank-and-file on Monday, lest Hillary Clinton have the ability to advance her party's legislative priorities and seat sympathetic Supreme Court justices without opposition.

It's notable that after reports he was mulling a full unendorsement of the Republican nominee, Mr Ryan is apparently trying to a walk a fine line between abandonment and loyalty to his putative standard-bearer. His decision evokes shades of 1996, when Republican nominee Bob Dole's doomed presidential campaign rolled along, oblivious to a party apparatus that was focusing exclusively on local races.

It's worth keeping in mind that while Mr Ryan is sounding the abandon-ship alarm, Donald Trump may not play the stoic captain watching from the bridge. He's shown no loyalty to a Republican establishment that never truly embraced him and may have no qualms with lashing out at erstwhile friend and foe alike in the campaign's final, turbulent days.

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Asked about the video in the debate, Mr Trump turned his fire on Mrs Clinton's husband, ex-President Bill Clinton, whom he described as "abusive to women". She refused to address the comments.

At least 38 senior Republicans - including senators, members of Congress, and state governors - have withdrawn their support since the video surfaced on Friday.

But late on Monday Republican National Committee Chairman Reince Priebus said "nothing has changed" in relation to the campaign.

"I think the issue [of the remarks about groping] was taken care of at the debate," he said, quoted by the Politico website, after a conference call with committee members.

'No blank cheque'

According to sources familiar with a conference call he held with congress members on Monday, Mr Ryan appeared to have accepted that Mrs Clinton would win the White House and wanted to make sure Republicans in Congress were strong enough to challenge her.

Mr Ryan said he would spend "his entire energy making sure that Hillary Clinton does not get a blank cheque with a Democrat-controlled Congress", the source said.

"You all need to do what's best for you in your district," he was quoted as telling colleagues.

He, however, faced some resistance from colleagues for his approach.

California Representative Dana Rohrabacher, called party leaders "cowards" during the call for distancing themselves from the Republican's official nominee, reports say.

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Mr Trump apologised for the remarks, and when pressed during the debate on whether he had engaged in sexual misconduct, he denied doing so.

But Mrs Clinton said his explanation that these were words not actions did not amount to an apology.

"If Trump stands by what he said about women as "locker room talk," he's clearly not sorry," she tweeted.

Meanwhile Mr Trump's running mate Mike Pence said he would stand by him despite the outcry over the remarks.

"I think last night he showed his heart to the American people. He said he apologised to his family, apologised to the American people, that he was embarrassed by it," he told CNN on Monday.

Earlier Mr Pence had described the remarks as indefensible.

The vice-presidential candidate said he was "honoured to stand with" Mr Trump and denied he had considered withdrawing from the race.

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