Floyd Mayweather Jr. calls out Manny Pacquiao via Twitter.

LAS VEGAS - Unbeaten US star Floyd Mayweather issued a challenge to boxing rival Manny Pacquiao on Tuesday, daring the Filipino icon to "Step up Punk" in a posting on Twitter.

Mayweather hopes to entice Pacquiao into a May 5 bout in Las Vegas, a mega-fight showdown between the world's top pound-for-pound fighters, one that boxing fans have wanted to see for years.

"Manny Pacquiao I'm calling you out let's fight May 5th and give the world what they want to see," Mayweather posted on Twitter.

The move comes as Pacquiao promoter Bob Arum is visiting his fighter in the Philippines to discuss who the Asian superstar should take on next and Mayweather awaits the June 1 start of a 90-day jail sentence on domestic violence charges.

Arum originally said that Mayweather was not among possible next foes for "Pac-Man", listing Mexico's Juan Manuel Marquez, Puerto Rican southpaw Miguel Cotto and Americans Tim Bradley and Lamont Peterson as candidates.

But that was assuming Mayweather was going to start serving his sentence last week. Instead, a judge postponed Mayweather's trip behind bars until after his already-booked May 5 fight.

"My Jail Sentence was pushed back because the date was locked in. Step up Punk," Mayweather also tweeted.

Mayweather dares Pacquiao to "step up."

Pacquiao, who has won 15 fights in a row over the past seven years, has told Filipino media that he wants the opportunity to fight Mayweather to be a top priority.

Mayweather, who squandered a chance to make a fight with Pacquiao earlier by seeking extraordinary blood testing measures, hopes to add pressure upon Arum and Pacquiao with his Twitter challenge.

But Arum has ripped Mayweather's May 5 plan, saying he could secure $40 million more with the construction of a temporary venue in Las Vegas to stage such a long-awaited fight before more than twice as many spectators.

"You don't throw $40 million away," Arum told the Los Angeles Times after speaking with Pacquiao, saying "June 9 is perfect" for a fight but "May 5 is out, that's impossible."

Arum said he was willing to help pay lawyer costs for Mayweather to delay the start of his sentence so a June 9 fight with Pacquiao could be staged.

"I want to hear... that they're willing to move to June 9," Arum said. "Then we can sit down and work out a deal."

Critics say Arum wants to avoid sending Pacquiao, 54-3 with two drawn and 38 knockouts, against Mayweather, 42-0 with 26 knockouts, in favor of deals with other fighters that he promotes, such as Bradley.

Past near-deals for Mayweather-Pacquiao have fallen apart.

But this time, both fighters are coming off controversial victories.

Pacquiao, a 33-year-old southpaw, won a majority decision over Marquez last November, giving him two wins and a draw over the Mexican but none of them by such a margin that Marquez's claims he was wronged could be dismissed.

Mayweather, who turns 35 next month, won the World Boxing Council welterweight title with a fourth-round knockout of compatriot Victor Ortiz, but he landed the decisive blow while Ortiz was distracted after the referee had separated the fighters then said they could resume.

Should Pacquiao not be next on Mayweather's hit list, "Pretty Boy Floyd" has been mentioned as filling the May 5 date with either US southpaw Robert Guerrero, 29-1-1 with 18 knockouts, or Mexico's Saul "Cinnamon" Alvarez, the WBC light-middleweight champion who is 39-0 with one drawn and 29 knockouts.