US actor Robin Williams was found dead Monday in his home in California, authorities said.

The mayor's office of California State's Marin county has been investigating what it said is a possible suicide of the 63-year-old Oscar winning actor-comedian, Efe reported.

According to his publicist, Williams had been "battling severe depression" in the period before his death.

The Marin County Sheriff's office, however, said that it had tentatively concluded that Williams died by suicide, specifically "asphyxia", although it provided no further details.

The star of many Hollywood comedies was found unconscious and not breathing at his home near San Francisco, by paramedics called to the house and he was pronounced dead at the scene at 12.02 p.m.

Williams starred in numerous Hollywood films, including "Good Morning, Vietnam", "Dead Poets Society", "Jumanji", "Mrs. Doubtfire" and "Good Will Hunting", winning the Oscar for Best Supporting Actor for the latter in 1997.

He also received two Emmy Awards, four Golden Globes, two Screen Actors Guild Awards and five Grammy Awards in addition to pursuing a successful decades-long and highly acclaimed standup comedy career.

Over the years, he had battled cocaine and alcohol addiction, as well as depression.

Authorities have opened an investigation into the actor's sudden death and an autopsy will be conducted Tuesday.