A fifth woman on Monday accused Republican Senate candidate Roy Moore of sexual misconduct, alleging he assaulted her when she was a teenager, as senior party members called on the conservative to withdraw from next month's election in Alabama.

Moore -- a former state supreme court judge and Christian evangelical running neck and neck with a Democrat in the race -- denies any wrongdoing.

But Beverly Young Nelson claimed the now 70-year-old Moore sexually assaulted her when she was a 16-year-old waitress and he was a county district attorney "twice my age" in around January 1978.

Nelson, who is now a 55-year-old businesswoman in Alabama, told reporters in New York that Moore was a regular customer in the restaurant where she worked and was flirtatious, signing her high school yearbook during one shift in late 1977.

A week or two later, Nelson said she was waiting for her boyfriend to drive her home after work, when Moore instead offered to give her a ride.

But instead of turning onto the highway, he drove behind the restaurant to a "dark and deserted" spot.

"Mr Moore reached over and began groping me, putting his hands on my breasts," she said, reading from a prepared statement alongside celebrity lawyer Gloria Allred, who has represented women who accused Bill Cosby and Harvey Weinstein of sexual assault.

"I tried fighting him off, while yelling at him to stop, but instead of stopping, he began squeezing my neck, attempting to force my head onto his crotch," she said.

"I was terrified. He was also trying to pull my shirt off. I thought that he was going to rape me. I was twisting and struggling and begging him to stop. I had tears running down my face," Nelson said.

He alleged that Moore gave up and drove off, leaving her behind "on the cold concrete in the dark."

- 'Frightened' -

Nelson said she was motivated to speak out after four other women told The Washington Post that Moore also pursued them in their teens, while he was in his early 30s working as an assistant district attorney.

Moore dismissed those allegations as a "dangerous lie."

Asked whether he remembered dating women in their teens when he was in his 30s, he replied: "Not generally, no."

But on Monday, he faced mounting calls to quit.

Even before Nelson spoke, the Senate's top Republican Mitch McConnell said he believed Moore's accusers and urged the anti-establishment conservative to "step aside" from the December 12 election.

The number two Senate Republican, John Cornyn, withdrew his endorsement of Moore, calling the accusations "disturbing and, if true, disqualifying."

Cory Gardner, the senator in charge of the 2018 Senate election campaign for all Republicans, said if Moore refused to withdraw and were to be elected, "the Senate should vote to expel him."

- Tight race -

Moore is locked in a tight race with Democrat Doug Jones in the contest to replace Jeff Sessions, now US attorney general, as Republicans seek to hold on to their slim 52-48 majority in the Senate.

Nelson batted aside any suggestion that she was motivated by politics.

"My husband and I supported Donald Trump for president," she said.

"Mr Moore attacked me when I was a child. I did nothing to deserve this sexual attack. I was frightened by his position and his power," she said.

Allred left open the possibility that other accusers may come forward, refusing to comment when asked if other women had contacted her about Moore.

The explosive Washington Post investigation saw one woman allege that Moore initiated a sexual encounter with her in 1979, when she was 14 years old and he was 32. Three others said Moore pursued them when they were 18 or younger.

White House officials have said that if the accusations are true, Moore should step aside and McConnell has floated the possibility of getting a Republican to come forward as a write-in candidate in the race.

Moore's controversial positions put him on the far right of the Republican Party.