A Japanese amateur boxer underwent brain surgery two weeks ago and has not regained consciousness, authorities said on Friday, as the sport in the country reels from the deaths of two professional fighters.

The Japan Boxing Federation (JBF) said the unnamed 39-year-old became unresponsive after sparring for three rounds at three minutes each in Tokyo on Aug. 8.

He was rushed to hospital and underwent emergency surgery, and is still in intensive care.

The JBF, which oversees amateur boxing in Japan, said the man had not fought a match for over 10 years and had been in training to make a comeback.

"We are praying he recovers as quickly as possible," said JBF president Tatsuya Nakama.

Japanese boxing is under the spotlight after super featherweight Shigetoshi Kotari and lightweight Hiromasa Urakawa, both 28, died days after injuries sustained in separate bouts in Tokyo on August 2.

Japanese boxing officials have held a series of emergency meetings and vowed to introduce new safety rules.

New measures to be implemented include urine tests for measuring dehydration and stricter rules on boxers' rapid weight loss.

Japanese boxer Yudai Shigeoka announced his retirement this month to support his brother, who has been in a coma since a bout in May.

Former WBC strawweight champion Shigeoka's brother Ginjiro collapsed after a fight in Osaka and underwent emergency brain surgery.

The 25-year-old remains in a coma but is no longer in a life-threatening condition, the Japan Boxing Commission says.