The ad was posted to the "free stuff" section of the site yesterday alongside offers for free turkey dinners, a set of crutches and an electric stove.

The writer, who said he was in San Diego, California, claimed that his ex-girlfriend had the baby a few weeks ago. "Now he just sits in my closet and cries, " the ad stated. "I'm not too sure how to deal with it."

The posting added: "Batteries not included. Transaction final. No returns."

Craigslist removed the ad shortly after it appeared.

"This is most probably a hoax, " site founder Craig Newmark said in an email to the Associated Press today.

San Diego police said the matter was referred to investigators in the child-abuse unit and another department that investigates internet crimes against children.

"We're taking it seriously because we don't know, " said police spokeswoman Monica Munoz.

"Until we determine whether or not it's a hoax or not we won't let it go."