Japan's prime minister apologised on Wednesday to victims of a now-defunct eugenics law that saw thousands of people forcibly sterilised between 1948 and 1996.

"The responsibility of the government as the enforcer of the eugenics law is extremely heavy," Fumio Kishida said at his office in Tokyo.

"I express my heart-felt apology, representing the government," he said, bowing to a group of victims.

Kishida also pledged to take new measures to ensure victims receive compensation in addition to a 2019 payment of 3.2 million yen (around $20,000 today) that campaigners said was too small.

The apology follows a July 3 ruling by the country's top court declaring the law unconstitutional and that a 20-year statute of limitations preventing some victims from receiving compensation could not be applied.

The government, which apologised in 2019, acknowledges that around 16,500 people including those with mental and physical disabilities were forcibly sterilised under the law that aimed to "prevent the generation of poor quality descendants".

Another 8,500 people were sterilised with their consent, although lawyers say they were likely "de facto forced" because of the pressure individuals faced.

A 1953 government notice said physical restraint, anaesthesia and even "deception" could be used for the operations.

The dark chapter in Japanese history was thrust back under the spotlight in 2018 when a woman in her 60s sued the government over a procedure she had undergone at age 15, opening the floodgates for similar lawsuits.

Following the top court ruling, a nonpartisan group of lawmakers has started studying new compensation, aiming to submit legislation to parliament later this year, Jiji Press reported.