Since early April, three Chinese vessels have been conducting marine research operations in the East China Sea, on the Japanese side of the midway line between Japan and China. These activities are based on a memorandum that the Japanese and Chinese governments exchanged in February. In accordance with the agreement, China gave advance notice of the operations to Japan and Japan approved the plan, the Japanese Foreign Ministry says. The Chinese vessels, however, are using pneumatic devices for acoustic exploration and conducting drilling tests, obviously outside the scope of the scientific research that Japan and China agreed to.

For the past several years, Chinese vessels have been conducting research activities in the same zone, defying Japanese protests. In 1999, a total of more than 30 vessels, sometimes accompanied by warships, conducted research operations in the waters. In 2000, a Chinese spy boat crossed the Tsushima Strait between Japan and South Korea and then the Tsugaru Strait in Japan, moving southward along the Pacific coast to waters off Cape Inubo in Chiba Prefecture. There the vessel reportedly conducted intelligence-gathering operations focused on metropolitan Tokyo before going back to China. These activities prompted Japan to exchange a memorandum with China about controlling research operations in the East China Sea.

The document requires Japan and China to give two months' notice through diplomatic channels on marine research activities in the East China Sea, which is known as Dong Hai in China. Japan must notify China about entering "waters near China," while China must notify Japan about entering "waters near Japan in which Japan has an interest." The latter wording appears to imply that Japan has no rights in the waters.