Women who have illegal abortions should be punished, Republican presidential frontrunner Donald Trump said Wednesday, in comments branded "horrific" by Democratic rival Hillary Clinton.

"There has to be some form of punishment," the billionaire reality television star said in excerpts of an interview to be broadcast later Wednesday, after MSNBC's Chris Matthews asked: "Do you believe in punishment for abortion, yes or no?"

"For the woman?" asked the host. "Yeah, there has to be some form," Trump responded, after calling for a "ban" on the procedure.

The 69-year-old Trump, who during his political life has been a Democrat and an independent and is only a recent convert to the "pro-life" anti-abortion position, has been accused in the past of flip-flopping on the hot-button issue of abortion.

Trump's televised comments caused an immediate outcry among pro-choice advocates -- but may also resonate with the section of US society that is virulently opposed to abortion.

Clinton wasted little time in voicing her disgust at his controversial remarks, just the latest in a series from a candidate already accused of peddling misogyny and anti-Islam sentiment on the campaign trail.

"Just when you thought it couldn't get worse. Horrific and telling," Clinton tweeted.

She and Democratic lawmakers are defenders of a woman's right to obtain an abortion, a medical procedure still fiercely opposed by many Republicans, four decades after the US Supreme Court affirmed its legality nationwide.

As the firestorm erupted around him, Trump moved to clarify his position.

The Trump campaign issued a statement backpedalling on his remarks, insisting that if abortion were to be banned by a US federal or state court, "the doctor or any other person performing this illegal act upon a woman would be held legally responsible, not the woman."

"The woman is a victim in this case as is the life in her womb. My position has not changed -- like Ronald Reagan, I am pro-life with exceptions."