Beijing (CNN) -- Two Chinese patrol ships have arrived near a disputed set of islands in the East China Sea that Japan has announced plans to bring under public ownership, Chinese state media said Tuesday, further ratcheting up tensions between the two Asian nations.
The state-run Xinhua news agency reported that the two vessels from China Marine Surveillance had "reached the waters around the Diaoyu Islands," which Japan calls the Senkaku Islands.
The marine surveillance agency has "drafted an action plan for safeguarding the sovereignty" of the islands and will "take actions pending the development of the situation," Xinhua reported, citing unidentified sources at the agency.
The Japanese government said Monday that it planned to acquire the group of small islands from the Kurihara family, a private Japanese owner. At a meeting Tuesday, it approved the purchase for 2.05 billion yen ($26.2 million), according to Chief Cabinet Secretary Osamu Fujimura.
The Chinese Foreign Ministry published a lengthy statement Monday detailing Beijing's arguments behind its claim of sovereignty over the islands, which are situated between Taiwan and Okinawa.
After insisting that the Japanese government's acquisition of the territory was "illegal and invalid," the statement appeared to issue a warning to Tokyo.
"Long gone are the days when the Chinese nation was subject to bullying and humiliation from others," it said. "The Chinese government will not sit idly by watching its territorial sovereignty being infringed upon."
The islands are at the heart of a bitter diplomatic argument between the two countries that has resulted in occasionally violent acts of public protest. The islands sit among popular fishing waters and are also believed to be rich in oil resources.
In April, Gov. Shintaro Ishihara of Tokyo launched an effort to raise money to acquire the islands for city authorities. The move set off a new cycle of tensions between Japan and China over which country has sovereignty over them.
Ishihara's effort prompted the government of Prime Minister Yoshihiko Noda to step in with its own bid for the islands.
On Tuesday, Fujimura tried to downplay the deal's significance for relations between Japan and China, portraying it as an internal real-estate transaction.
"We don't want for this matter to adversely affect the bilateral relationship," Fujimura said.
"Both governments have been communicating closely through diplomatic channels to avoid misunderstandings and unforeseen incidents," he added.
The Japanese Coast Guard has not confirmed the presence of the Chinese surveillance vessels in the vicinity of the islands, said an official at the 11th Regional Coast Guard Headquarters in Okinawa, who gave only his family name of Fukumine.
China has sent ships near the islands previously. In July, three Chinese fishery patrol ships "engaged in a verbal confrontation" with Japanese Coast Guard ships in the area, according to Xinhua.
Animosity between China and Japan over the islands runs deep.
They have come to represent what many Chinese people see as unfinished business: redressing the impact of the Japanese occupation of large swathes of eastern China during the 1930s and 1940s.
China says its claim goes back hundreds of years. Japan says it saw no trace of Chinese control of the islands in an 1885 survey, so formally recognized them as Japanese sovereign territory in 1895.
Japan then sold the islands in 1932 to descendants of the original settlers. The Japanese surrender at the end of World War II in 1945 only served to cloud the issue further.
The islands were administered by the U.S. occupation force after the war. But in 1972, Washington returned them to Japan as part of its withdrawal from Okinawa.
CNN's Shao Tian in Beijing and Junko Ogura in Tokyo contributed to this report.