At least 19 Japanese websites, including those of a government ministry, courts and a hospital, have come under cyberattack, apparently from China, police said Wednesday.

Many of the websites were altered to show messages proclaiming Chinese sovereignty over the Diaoyu islands, a Japanese-administered chain Tokyo calls Senkaku, the National Police Agency (NPA) said in a statement.

The NPA has confirmed that about 300 Japanese organisations were listed as potential targets for cyberattack on the message board of Honker Union, a Chinese "hacktivism" group, it said.

The police also confirmed around 4,000 people had posted messages about planned attacks and schemes on China's leading chat site "YY Chat", it said.

The targeted sites include those of the Internal Affairs and Communications Ministry and Tohoku University Hospital, police said.

The website of the ministry's statistics bureau seemed to have come under a "distributed denial of service (DDoS)" attack, where huge volumes of data are sent in a short period to paralyse the targeted server, Kyodo News said.

On Sunday afternoon, when the attack was most intense, 95 percent of traffic to the bureau's website was from China, Kyodo said, citing minister Tatsuo Kawabata.

Beijing and Tokyo have been locked in an intensifying spat over the uninhabited but strategically important outcrops in the East China Sea.

The dispute has been rumbling for decades, but took a sudden lurch for the worse when Japan bought three of the islands from their private Japanese owner this month.