Three Chinese fishery patrol ships entered waters near the Senkaku islands, disputed by Japan and China, prompting a protest from the Japanese authorities.

The islands, which Beijing calls Diaoyu, are located near rich fishing grounds and may contain important gas and oil reserves.

The three Chinese ships eventually left the waters, but two of them remained in the nearby area, being closely watched by Japanese patrol ships around 10.30 a.m. (01.30 GMT) on Wednesday, said chief cabinet secretary Osamu Fujimura in a press conference.

Chinese news agency Xinhua said the patrol vessels had entered the waters “to carry out a fishery protection mission in our exclusive economic zone” and repeated that the islands and surrounding waters have been Chinese territory since ancient times.

Prime minister Yoshihiko Noda stated last weekend that Japan is considering buying the Senkaku islands, fuelling the diplomatic controversy between the two countries.

“There is no question that the Senkakus are an integral part of our country’s territory in terms of history and international law,” Noda said on Saturday. “From the viewpoint of how to maintain and manage the Senkakus in a calm and stable manner, we are making comprehensive studies on the matter by keeping in touch with the owner.”