Shocking footage shows children having their stomachs pummelled with repeated powerful punches while undergoing controversial Muay Thai 'strength training'.

A little boy wore gloves while he hit a girl on the stomach during a sparring session in Nakhon Si Thammarat province, Thailand, on February 21.

He did not stop, even though the little female boxer scrunched up her face in pain from her friend's powerful jabs to the belt area of the shorts.

The kid also punched his other friends continuously in the stomach to practice their endurance from pain in the training camp. The little fighters then switched positions to be the ones administering the blows.

Their trainer said it toughens up the kids to prepare them for a career in Muay Thai - an industry worth more than half a billion dollars annually.

While supporters argue Muay Thai teaches kids discipline and self-defence when done properly, critics worry about the potential physical and psychological impacts on children exposed to a combat sport.

Other Muay Thai camps have allegedly exploited child fighters for profit by making them compete in dangerous bouts at a very young age before their bodies are ready.

The popularity of Muay Thai, which originated in the mighty Khmer Empire, has soared in recent years in line with the rise in MMA and cage fighting. Thai boxing is now popular in Japan, South Korea, Russia and the United States.