Los Angeles - Robert Downey Jr is keeping fans guessing whether there is going to be a fourth Iron Man movie, hours after he told Ellen DeGeneres at a taping of her show that it is on.

On Tuesday night, the Iron Man star appeared to have backtracked on his earlier comment, telling David Letterman on his show, "There's no plans for an Iron Man 4."

On The Ellen Show, airing on Thursday, he had reportedly said, "We're in the middle of negotiations," before finally confirming the sequel and telling the comedienne, "Okay, yes".

But on Tuesday night, he told Letterman on Late Show that there is no script, reported the Daily Mail.

Asked if he was going to "make another one of those Iron Man movies", he replied, "Just between us, no.

"But I'm going to do other stuff with Marvel. I'm still going to be involved with Marvel, and there's going to be plenty of fun stuff to happen."

His latest comments have had some in the media saying there might be a new Iron Man star, or that the new movie might involve Iron Man with other superheroes.

Last month, the actor said there is "no plan" for another Iron Man movie while promoting his film The Judge at the Toronto Film Festival.

Last week, he told move website Deadline that he would be on board for the sequel if his friend Mel Gibson was tapped to direct.