LONDON (GUARDIAN) - American MMA superstar Ronda Rousey has escalated her public war of words with Floyd Mayweather by saying she would beat him in a 'no-rules' fight.

"Floyd is one of the best boxers of all time," Rousey wrote during a Reddit Ask Me Anything session .

"He would definitely beat me in a boxing match. I unfortunately don't get into 'matches.' I fight for a living.

"In a no-rules fight, I believe I can beat anyone on this planet," she said.

"Boxing is a sweet science with strict rules that I respect very much and aspire every day to improve at. But you said ruleless fight, and that's my honest answer."

The 28-year-old Rousey is the undefeated UFC female bantamweight champion and was named the female athlete of the year at this year's ESPYs.

Mayweather, 38, the five-division world champion, will take on fellow American boxer Andre Berto next month seeking to extend his unbeaten streak to a stunning 49-0.

Rousey was angered in July 2014 when Mayweather claimed he didn't know who she was, something Rousey said was untrue.

"He was running against me for Best Fighter that year, and he had signed MMA fighters, so his feigned ignorance was meant to really be a snarky remark to me," she said at the time.

In an interview with the Guardian on Monday , another UFC champion, Conor McGregor, expressed similar sentiments to Rousey.

"In a real fight I would dismantle (Mayweather) in seconds," said McGregor. "Ronda would dismantle him in seconds - 100 per cent.

"When you don't know how to grapple you don't stand a chance. For us, it's like playing with a baby. People who don't understand the sport cannot understand how vulnerable they are.

"There are always clinches and Ronda is a judo Olympian. She would throw Floyd on his head in a second. Me? I would knock him out. Cold."