Charlie Sheen, the wayward star of U.S. television comedy "Two and A Half Men," said on Tuesday he was diagnosed as HIV positive some four years ago and had been extorted for more than $10 million to keep the information quiet.

Sheen, 50, told NBC's "Today" TV show he was speaking out because he was being blackmailed, and to refute tabloid reports that he has AIDS and was spreading it to others.

"I have to put a stop to this onslaught, this barrage of attacks and of subtruths and very harmful and mercurial stories," he said.

"I am here to admit that I am in fact HIV positive," Sheen said, adding he was "not entirely sure" how he contracted the virus.

Sheen acknowledged he had paid "upwards of $10 million" in recent years to "desperate charlatans" for their silence about his condition.

The actor, who is three times divorced and has dated many porn stars, said it was "impossible" that he had transmitted the virus to anyone else. He said he "always led with condoms and honesty when it came to my condition."

But former porn star Bree Olson, one of the "goddesses" who lived with Sheen for a year four years ago, said on Tuesday he had never informed her of his HIV status.

"Never said anything. Ever. 'I'm clean,' he told me. 'I'm clean, I'm clean'," Olson told Howard Stern in a radio interview on Tuesday. Olson, who said she has since tested negative for HIV, made similar statements to entertainment TV show "Inside Edition."

Sheen's spokesman declined to comment when asked about Olson's remarks.

Sheen played the womanizing bachelor Charlie Harper on top-rated comedy "Two and A Half Men" for eight years before being fired in 2011 for bad behaviour that included cocaine-fuelled partying and assaulting an ex-wife.

At the time, he was the highest paid actor on U.S, television, with a reported salary of some $1.8 million per episode.

He said he informed his ex-wives, actresses Brooke Mueller and Denise Richards, immediately after getting his diagnosis. Mueller and Richards both issued statements saying neither they nor their four children with Sheen were HIV positive.

Sheen's doctor, Robert Huizenga, said the actor "does not have AIDS," that he is taking antiretroviral drugs and that his viral HIV load was now "consistently undetectable."

"ABYSMAL DESCENT"

After being fired from "Two and A Half Men," Sheen set up home with Olson and another porn star and boasted of having "tiger blood" in his veins.

It was around that time that Sheen was first diagnosed.

Sheen said shame and disbelief over the diagnosis led to "an abysmal descent into profound substance abuse and fathomless drinking. It was a suicide run."

The actor said he has now put his partying days behind him.

Sheen, the son of "West Wing" TV actor Martin Sheen, said he doesn't feel any stigma from being HIV positive, a condition that affects more than 1.2 million people in the United States alone, according to government statistics.

HIV, or human immunodeficiency virus, weakens the immune system. No cure exists, but with medical care it can be controlled. If not, one can develop AIDS.

Reaction to Sheen's announcement was largely compassionate, and his name quickly became the top trending topic on Twitter.

"Total respect to @charliesheen opening up about his HIV positive status, may your strength be repaid and know people really do care!" tweeted Brett Herriot.

Another Twitter posting, from Howarse7, said, "I don't condone Charlie Sheen's recklessness but the headlines and jokes about it show just how little society has progressed. It's sad."

Sheen also starred in 1980s movies "Platoon" and "Wall Street," as well as TV comedies "Spin City" and "Anger Management."