US Comedian Bill Cosby is suing seven women who accused him of assaulting them, claiming they defamed him.

They are among 50 who have come forward to accuse the actor of sexually assaulting them after plying them with drugs and alcohol.

In the countersuit, filed in US district court in Springfield, Massachusetts, Cosby says the seven lied when they made their accusations.

His suit is in response to lawsuits filed by all seven women charging him with libel, assault and slander and seeking unspecified financial damages.

The 78-year-old, best known for his role in the 1980s television hit 'The Cosby Show', has repeatedly denied doing anything wrong and has never been criminally charged.

Many of the assaults he is alleged to have committed occurred decades ago so the statute of limitations for prosecuting them has elapsed.

The countersuit contends that the women defamed his "honourable legacy and reputation" by accusing him of sexual misconduct.

It says: "Relying solely on unsubstantiated accusations, counterclaim defendants have engaged in a campaign to assassinate Mr Cosby's reputation and character by willfully, maliciously, and falsely accusing Mr Cosby of multi-decade-old purported sexual misconduct".

It charges the women with defamation, tortious interference and intentional infliction of emotional distress.

The women's lawsuits were first filed in December 2014.

Gloria Allred, who represents several other women suing the comedian, said: "Such a tactic will not deter courageous women from fighting the battle against him".

The cases are all ongoing.