A US talk show host who mocked Joaquin Phoenix's supposed "cleft lip" has apologized after parents of children with the condition slammed her "bullying" behavior.

Wendy Williams, host of "The Wendy Williams Show," pulled up her lip with her finger for several seconds while joking about how she found Phoenix's scar attractive.

"When he shaves off his moustache he's got a hairline fracture -- he's got one of those, what do you call it, cleft lip? Cleft palate," she said, to laughter, during a show last week.

"Oh, I find it to be, I find it to be very attractive."

The imitation drew a furious response from parents including professional Canadian football player Adam Bighill.

Phoenix said that he had a "non-surgical scar he was born with," not a "surgically fixed cleft" / © GETTY IMAGES NORTH AMERICA/AFP/File

"@WendyWilliams comments/actions towards the cleft community clearly promotes bullying," tweeted Bighill, adding that Williams's actions were "hideous and offensive."

Bighill, who himself was born with a cleft lip, also tweeted a picture of his son Beau, shortly before the child went into surgery to repair the condition.

A cleft is a gap in the mouth that didn't close during pregnancy, and affects about one in 500-750 births worldwide, according to medical non-profit Operation Smile.

Phoenix told Vanity Fair last year that he had a "non-surgical scar he was born with," not a "surgically fixed cleft."

Williams pledged to donate to cleft lip organizations and tweeted Wednesday: "I want to apologize to the cleft community and in Beau's honor."