US comic Bill Cosby has been charged with sexual assault in Pennsylvania over an incident that took place in 2004, a prosecutor said on Wednesday - the first criminal charges filed against the actor after dozens of women claimed abuse.

"Mr Cosby is charged with aggravated indecent assault. This is a felony," Kevin Steele, assistant district attorney for the state's Montgomery County, told reporters in Norristown, in the suburbs of Philadelphia.

Cosby, 78, was to be arraigned later on Wednesday. If found guilty, the veteran television star could face up to 10 years in prison and a $25,000 fine.

More than 50 women have already publicly claimed abuse by the pioneering African-American comedian, who attained his greatest fame for his role as the lovable family doctor and pater familias in the hit 1980s sitcom The Cosby Show.

But his attorneys have repeatedly denied any wrongdoing by Cosby, who has gone from megastar to pariah in the face of the allegations, which span four decades. Earlier this month, he filed a defamation suit against seven of his accusers.

The Pennsylvania criminal charge stems from allegations made by a former employee of Temple University in Philadelphia, who says the television legend assaulted her at his home in the suburb of Cheltenham in 2004.

"On the evening in question, Mr Cosby urged her to take pills that he provided to her and to drink wine - the effect of which rendered her unable to move, (or to) respond to his advances, and he committed aggravated indecent assault upon her," Steele said.

Steele said the charge - which prosecutors later clarified is a felony of the second degree - stemmed from new evidence in the case that came to light in July, prompting the reopening of the investigation.

"A prosecutor's job is to follow the evidence wherever it leads and whenever it comes to light," Steele said.

"Today, after examination of all the evidence, we are able to seek justice on behalf of the victim," he added, urging anyone with information about similar incidents with Cosby to come forward.

Statute of limitations  

Steele did not name the victim, but the facts of the case correspond with allegations made by Andrea Constand. Constand's lawyer, Dolores Troiani, confirmed to CNN that the charge was connected to her.

In 2005, an initial investigation did not result in criminal charges against Cosby in Constand's case.

In a deposition Cosby gave as part of a civil suit filed by Constand, which was settled in 2006, the actor insisted all relations with Constand were consensual, and accused her of lying about the assault.

"I think Andrea is a liar and I know she's a liar because I was there. I was there," Cosby said in the deposition, cited by The New York Times.

Prosecutors said the release of court documents in July and the subsequent release of the deposition had in part led to the reopening of the case. Steele said the victim had said she was willing to cooperate with prosecutors.

Statutes of limitations have prevented most of the women who have accused the comedian of sexual assault from taking legal action. In the Pennsylvania case, the statute of limitations was set to expire early next year.

The accusations against Cosby have led television networks to back away from projects connected to him, and several universities have stripped him of honorary degrees.