Japan's health, labour and welfare ministry's website has come under a cyber attack and remains inaccessible, the media reported on Saturday.
The international group of hackers who call themselves "Anonymous" claimed they launched the attack on Friday night, public broadcaster NHK reported.
A message posted on Twitter suggests Anonymous attacked the website by overhauling it with massive amounts of data -- a tactic called a distributed denial-of-service attack (DDoS).
Ministry officials are investigating the Twitter post and are working to restore service.
Anonymous have attacked Japanese companies including a newspaper, as well as airport and railway operators, since September.
