The Republican challenger for a US Senate seat has said he "misspoke" when he said women have biological defences to prevent pregnancy in cases of "legitimate rape", making abortion rights unnecessary.

US Representative Todd Akin, who is running against Democratic Senator Claire McCaskill in the 6 November election in Missouri, made the comments in St Louis, drawing swift protest from Ms McCaskill and other Democrats.

Mr Akin said in the interview that the need for abortions in the case of rape was "a particularly tough ethical question".

"It seems to me, from what I understand from doctors, that is really rare," Mr Akin, a Tea Party-backed conservative, said of pregnancy caused by rape.

"If it's a legitimate rape, the female body has ways to try to shut that whole thing down," Mr Akin said.

"But let's assume that maybe that didn't work or something: I think there should be some punishment, but the punishment ought to be of the rapist, and not attacking the child."

Ms McCaskill, who is considered one of the most vulnerable Senate Democrats in a state that has shifted to the right since she was first elected in 2006, fired off a rebuke on Twitter.

"As a woman and former prosecutor who handled 100s of rape cases, I'm stunned by Rep Akin's comments about victims this AM," she wrote.

Mr Akin later backtracked, saying in a statement: "In reviewing my off-the-cuff remarks, it's clear that I misspoke in this interview and it does not reflect the deep empathy I hold for the thousands of women who are raped and abused every year."

He promised that rape victims "will have no stronger advocate in the Senate to help ensure they have the justice they deserve" if he was elected.

However, Mr Akin did not back down on his position opposing abortion in the case of rape, adding: "I believe deeply in the protection of all life, and I do not believe that harming another innocent victim is the right course of action."

Republican presidential candidate Mitt Romney's campaign distanced itself from Mr Akin's position.

"Governor Romney and congressman (Paul) Ryan disagree with Mr Akin's statement, and a Romney-Ryan administration would not oppose abortion in instances of rape," the statement read.

Mr Akin, a six-term congressman from the St Louis suburbs, won a hard-fought three-way primary race two weeks ago.

Ms McCaskill's campaign spent funds during that campaign to help him win, viewing him as a far-right conservative who would be easier to defeat in November.