UFC star Conor McGregor said in a new promotional video that he will “break” Floyd Mayweather’s face when the two fighters contest a boxing rules showdown in August.

The Las Vegas mega fight brings together two of the biggest names in combat sports.

McGregor – a two-time two-weight champion in mixed martial arts (MMA) – challenges Mayweather, an unbeaten boxer who has ruled his sport for a 20 year period.

“I’m going to be ruthless in there,” McGregor says in the video. “My fist is just going to break his whole face and that’s it.”

The video coincides with the launch of a world press tour. This week, McGregor and Mayweather will meet face-to-face for the first time since they announced their half a billion dollar bout in June.

Both fighters are renowned for their mastery of self-promotion, so the tour, which begins in Los Angeles on Tuesday, is bound to generate headlines.

Even the official trailer, produced by American broadcaster Showtime Sports, has some immortal lines.

"Let's not kid ourselves here who the big star is of this show," McGregor says. "[I'm] a slick, Irish, handsome kid who has climbed to the top of the game quicker than anybody else has ever done it."

"I'm always looking to make history… and here we are again, more history."

Mayweather counters McGregor's "big star" claim. "When you speak boxing, you're speaking Mayweather."

He added: "This is not like a normal fight. It's big, big business."

You can watch Showtime's promo video here.