Japan's female Olympic judo athletes were beaten with bamboo swords and slapped by their coaches, officials said Wednesday, weeks after a schoolboy's suicide sparked anguished debate over corporal punishment.

A 15-strong group of judokas complained to the Japanese Olympic Committee (JOC) last month that they had been subjected to physical punishment by the team's head coach.

The group, which included athletes who took part in the London Olympics, says head coach Ryuji Sonoda routinely abused them, slapping them in the face and hitting them with thick wooden swords, like those used in the Japanese martial art of kendo.

They also complained that some were forced to compete in matches while injured, reports said.

"We have asked the All Japan Judo Federation (AJJF) to investigate the case and improve their methods if the charges are true," a JOC official said.

AJJF head Koshi Onozawa said the federation had admonished Sonoda and other coaches, who had admitted several of the allegations.

"We received information that Mr Sonoda, the head coach of the female national team, might have been physically bullying athletes," Onozawa told a news conference.

"Our executive office took this seriously and questioned both him and athletes, discovering the charges were largely true."

The AJJF told Sonoda and other coaches they must mend their ways and "will face a harsher punishment if a similar incident happens in the future", he added.

Kyodo News said Sonoda did not deny the allegations when asked by reporters.

"Until now I have been doing things the way I saw fit, but I will mend the things that need fixing," it quoted him as saying.

Japan's women returned from London with one gold, one silver and one bronze medal in judo, well below their haul from the 2008 Beijing Games.

JOC secretary general Noriyuki Ichihara told reporters the matter was not closed.

Asked if the AJJF's decision to keep Sonoda as head coach was appropriate, Ichihara said: "We want to see if the trust between athletes and coaches is still there or if there is a way to rebuild that trust," adding the AJJF has authority to appoint coaches.

The case comes weeks after a Japanese high school student killed himself after repeated physical abuse from his basketball coach, an incident that has provoked national hand-wringing over the way children are disciplined.

Under a law dating from 1947, teachers are not permitted to physically discipline their charges. However, there are no statutory penalties for the minority of teachers who do so.

It is not the first time Japan's sporting world has been rocked by violence.

In 2007 a trainee sumo wrestler died after a hazing incident revealed a shocking level of punishment for would-be champions.

Referring to Wednesday's claims, education and sports minister Hakubun Shimomura told reporters a rethink was required.

"It is time for Japan to change the idea that use of violence in sports including physical discipline is a valid way of coaching," he said.

Tomohiro Noguchi, a specialist in sport at Nihon University, said it was quite surprising that this kind of thing was happening at Olympic level.

"Mainstream ideas have shifted over a generation to advice-based, athlete-centred coaching," said Noguchi, 46, a former swimmer who said he himself was beaten by his coach as a teenager.

"But there are still some coaches who were physically punished in their youth who apparently still believe in the old method. We may have to look at how coaches are educated in sports science universities to prevent a repeat," he said.