US President Donald Trump would expect Senate candidate Roy Moore to withdraw from an upcoming vote if allegations that he had a sexual encounter with a 14-year-old girl were true, the White House said on Friday.

Senior Republicans are calling for the controversial Alabama Republican to pull out of the race to be elected to the Senate in December, after The Washington Post reported that he initiated a sexual relationship with a 14-year-old girl when he was 32.

Roy Moore, a 70-year-old judge, is hoping to take the seat vacated by Jeff Sessions, the attorney general.

Mr Moore was backed by Steve Bannon, President Donald Trump’s former chief strategist, and joined on the campaign trail by Nigel Farage. Mr Trump and the Republican establishment preferred his rival, Luther Strange.

A defiant Mr Moore emailed his supporters hours after the accusations were made and wrote: "I refuse to stand down."

He said that the accusations were attacks from "the Obama-Clinton Machine's liberal media lapdogs".

He continued: "The forces of evil are on the march in our country.

"I have a duty to stand up and fight back against the forces of evil waging an all-out war on our conservative values."