Japan has increased surveillance around disputed East China Sea islands, after Chinese ships made a rare prolonged visit to the waters around them.

Tokyo controls the islands and refers to them as Senkaku, while Beijing considers them to be Chinese territory and calls them Diaoyu.

Japan said four Chinese surveillance ships entered the waters around the disputed islands on Monday and remained for 13 hours, ignoring calls by the Japanese coast guard to leave. Japanese chief Cabinet secretary Yoshihide Suga called the prolonged Chinese patrol “extremely unusual” and “regrettable.”

The Japanese government also summoned the Chinese ambassador in Tokyo and demanded that China prevent a repeat of Monday's incident.

In Beijing, the Chinese foreign ministry said the Chinese vessels were conducting “normal” surveillance activities. It also accused Japan of repeatedly violating Chinese sovereignty by using vessels and planes to patrol the disputed islands over Beijing's objections.

The islands are surrounded by rich fishing grounds and potential energy deposits. The sovereignty dispute has strained relations between Japan and China for decades.

China has sent vessels to the area several times a month since September to assert Chinese claims after the Japanese government purchased three of the five disputed islands from a Japanese family. Several Chinese surveillance planes also have flown toward the archipelago in recent weeks, prompting Japan to scramble F-15 fighter jets to intercept them.

Japan annexed the uninhabited islets in the late 19th century. China claimed sovereignty over the archipelago in 1971, saying it has been Chinese territory for centuries.