Conservative Arizona Republican Representative Trent Franks is to resign from Congress, amid accusations of inappropriate behaviour, two GOP strategists said on Thursday, making Franks the third lawmaker to announce his resignation this week.

Neither Republican operative said they knew why the eight-term Franks was leaving. His departure had not been expected, and one of the consultants said Franks was leaving on Thursday.

Franks was seen being comforted in the chamber by several other Republicans shortly before he left the House floor. They included Texas Rep. Louie Gohmert, Alabama's Robert Aderholt and fellow Arizonan Andy Biggs.

Asked for comment as he left the floor, Franks said, "I'll let the statement speak for itself." But it was not clear what statement he was referencing.

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Franks has been a member of the far-right House Freedom Caucus. He's a staunch social conservative who sponsored House-passed legislation to make it a crime for any person to perform an abortion if the age of the foetus is 20 weeks or more.

The Republicans spoke on condition of anonymity to describe a decision they'd been told privately.

Several men in US politics, media and entertainment have been accused of sexual harassment and misconduct in the last few months.

Earlier, liberal Democrat Senator Al Franken, of Minnesota, announced his resignation after facing allegations of sexual harassment by at least eight women. Franken said some of those accusations were false and said he remembered others differently than his accusers did. He said he'd depart in a few weeks.

Rep. John Conyers, Democrat, Michigan, resigned effective immediately. He also faced accusations from women of improper sexual behaviour that he's contesting.

Franks drew a sharp response from Democrats during a 2013 House committee debate when he said "the incidence of rape resulting in pregnancy are very low". He sought to clarify the comment, saying later-term abortions linked to pregnancies caused by rape are infrequent.

He's a strong backer of President Donald Trump and has embraced some of his stances on social issues. Franks has harshly criticised some NFL players for not standing during the national anthem, calling them "arrogant and overpaid Lilliputians who dishonour America".

Franks represents a district encompassing suburbs north and west of Phoenix. He serves on the House Judiciary and Armed Services committees.

Before winning election to Congress, he served in the Arizona legislature and founded the Arizona Family Research Institute, an organisation associated with Dr James Dobson's "Focus on the Family". The institute advocates for policies designed to protect children and families.

- AP