Japan summoned the Chinese ambassador in Tokyo Thursday to express concern after a Chinese ship sailed close to disputed islands in the East China Sea.

“Around 00:50 am (11:50 a.m. EDT Wednesday), a Chinese naval vessel entered our nation’s contiguous waters surrounding the Senkaku Islands,” the Japanese Foreign Ministry said in a statement. Japan’s Vice Foreign Minister Akitaka Saiki summoned China’s ambassador Cheng Yonghua to lodge a protest.

Saiki told the ambassador that the islands are an inherent part of Japan's territory, in terms of history and international law, reported BBC. He also demanded the vessel move away from the area immediately. The Chinese vessel left the waters about an hour later, Reuters reported the Japanese Defense Ministry as saying.

Japan administers the uninhabited Senkaku islands that are also claimed by China, which calls it Diaoyu islands. Relations between the two countries took a turn for the worse after Japan “nationalized” the islands in 2012.

China's Defense Ministry said it was looking into Japan’s claims but asserted its freedom of movement within Chinese waters.  “Chinese naval ships sailing through waters our country has jurisdiction over is reasonable and legal. No other country has the right to make thoughtless remarks about this,” the ministry said, according to a statement sent to Reuters.

Japanese authorities also said that at around the same time, three Russian battleships entered waters close to what Japan considers its territory. They are now investigating whether the two actions were related. None of the ships violated the Japanese territorial waters, the country's defense ministry said.

Though Chinese patrol ships occasionally sail close to or enter these waters, this was the first time that a naval ship made its way into the disputed waters. The importance of the islands lies in their proximity to key shipping lanes and potential oil and gas reserves, as well as an abundance of rich fishing grounds.

In the past, China has warned Japan against taking any provocative action over the islands whereas Japan has told China that any foreign naval vessel that enter Japanese waters for reasons other than “innocent passage” would be told to leave by a Japanese naval patrol.