Legendary film director Woody Allen has hit back at his adopted daughter's allegations of sexual abuse during her youth, calling them "untrue and disgraceful."

Dylan Farrow broke her silence for the first time Saturday on the alleged abuse, saying it started when she was as young as seven.

"He told me to lay on my stomach and play with my brother's electric train set. Then he sexually assaulted me," Ms Farrow, 28, wrote in an open letter on a New York Times blog.

The unproven allegations first emerged more than two decades ago in the aftermath of Allen's acrimonious split with Mia Farrow in 1992.

"Mr Allen has read the article and found it untrue and disgraceful," Allen's publicist Leslee Dart said in a statement, adding that the director would be responding "very soon."

Ms Dart stressed that no charges were ever filed in the case and that an investigation by court-appointed independent experts at the time found no evidence of wrongdoing.

"The experts concluded there was no credible evidence of molestation, that Dylan Farrow had an inability to distinguish between fantasy and reality, and that Dylan Farrow had likely been coached by her mother Mia Farrow," Ms Dart said.

Mr Allen, 78, who left Mia Farrow after starting a relationship with the actress's adopted daughter from a previous marriage, Soon-Yi Previn, has always vigorously denied abusing Dylan Farrow.

A New York judge in the 1994 custody battle between Allen and Farrow ruled that the abuse allegations were inconclusive, but at the same time lambasted the director as "self-absorbed, untrustworthy and insensitive."

The younger Farrow accused the Hollywood establishment of sweeping Mr Allen's alleged crimes under the carpet by continuing to honour his films.