China is defending the actions of two of its patrol boats to enter disputed waters near Japan, saying the vessels were protecting Chinese fisherman.

Japan formally protested to China on Wednesday after the patrol boats entered what Japan considers its territorial waters in the East China Sea near an island chain known as Senkaku in Japan and Diaoyu in China. Japan administers the islands.

A Chinese foreign ministry spokesman said Wednesday the boats were on a routine patrol mission and he reiterated China's claim of the islands.

China's state-run media quoted the spokesman as saying the islands have been an integral part of China since ancient times. He said China has made its stance to Japan clear.

The incident Wednesday prompted Japan to summon the Chinese ambassador in Tokyo. Japan's Chief Cabinet Secretary Yukio Edano said the ambassador was told the islands are an integral part of Japanese territory, historically and under international law.

Japan's arrest of a Chinese fishing boat captain in the same area last year created a crisis in relations with China that lasted for several months.

The Japanese coast guard said the two fisheries patrol boats approached the islands Wednesday morning and sailed inside of what Japan claims as its 20-kilometer territorial limit.

The coast guard said it radioed a warning to the Chinese boats, which responded that they were acting legally within Chinese territory. It said they stayed for about 40 minutes.

Edano said Chinese vessels have approached the islands several times since last year's incident, but this was the first time they have sailed inside the 20-kilometer zone.

Last year's crisis was sparked when the Japanese arrested a Chinese fishing captain who rammed his trawler into a coast guard vessel.

China responded by breaking off all high-level contact with Japan and restricting the export to Japan of vital rare earth minerals used in high-tech manufacturing.