Hollywood icon Clint Eastwood could make Mitt Romney's day Thursday if he is the surprise speaker at the Republican National Convention, as event buzz and US media have led on.

Anonymous GOP sources quoted by Fox News and other US media said Wednesday the Oscar-winning director/actor and all-around tough guy will be the "mystery speaker" with a prime-time slot just before Senator Marco Rubio introduces Republican presidential nominee Mitt Romney.

Fox reported that a Republican source told its website that Eastwood is indeed the mystery man, but it did not name or quote any official in its report.

Conservative website Townhall cited what it called a well-placed Republican source as saying the "Dirty Harry" star, now 82, will travel to Tampa, Florida either Wednesday night or Thursday to attend the convention.

Eastwood's agent Leonard Hirshan said he was unaware of any such plans by his client.

"I have no knowledge of that fact.... I get him pictures, that's my job," he told AFP. "I'm his agent, but his personal life is his own. And whatever he does, I hear about it like you do, after the event.

"Maybe the Republican convention people know."

AFP requests for comment sent to convention organizers went unanswered.

Eastwood, whose next movie, "Trouble With The Curve," is set for release in September, endorsed Romney earlier this month, and backed the Republican nominee, Senator John McCain, in 2008.

He taped a high-profile commercial this year for Chrysler -- one of the companies saved in Obama's auto bailout -- that aired during the Super Bowl, leading to speculation that Eastwood might endorse Obama in 2012.

But the actor shot down any political association with Obama shortly after the ad aired, saying it was meant as "a message about job growth."

The spot ahead of Rubio and Romney on the official schedule reads "To Be Announced," and organizers on Tuesday heightened the sense of intrigue.

"Tune in!" campaign strategist Russ Schriefer challenged.

With most party heavyweights already securing spots during three nights of heady political theater 10 weeks out from the November election, speculation was rife that the mystery person could be a notable absentee.

Former presidents George and George W. Bush have both said they won't be attending, although a video from the pair was played to convention delegates on Wednesday evening.

Perhaps the biggest Republican celebrity not due to attend is former Alaska governor Sarah Palin, who lit up the 2008 convention as McCain's surprise running mate with her talk of hockey moms and lipstick.

A surprise 2012 convention appearance by the Tea Party favorite would certainly be a show-stopper, but pundits considered it highly unlikely as it would risk overshadowing Romney's big moment.