Six Chinese surveillance ships have sailed into waters around disputed islands in the East China Sea, the latest move in a bitter territorial dispute between China and Japan.
The Chinese Foreign Ministry said the ships were carrying out a mission of "law enforcement over its maritime rights" over the islands -- known as the Diaoyu in China and Senkaku in Japan.
The Japanese coast guard said they ordered the six ships to leave Japan's territorial waters, but only two or three complied.
No force had been used to remove the ships.
Japanese Prime Minister Yoshihiko Noda said, "we'll do our utmost in vigilance and surveillance," while Chief Cabinet Secretary Osamu Fujimura said Tokyo would "take all necessary measures."
"It is clear that the Senkaku [Islands] are part of Japan's sovereign territory, both historically and under international law, and they are also under the effective control of Japan," Fujimura added.
"It is extremely regrettable that China entered our sovereign waters in this way and we strongly advise China to withdraw from the area."
The developments come days after Tokyo sealed a deal to buy three of the islands from their private Japanese owner.
Japanese In China Warned
China's Defense Ministry has stated that the government and armed forces were "unshakeable in its determination and will safeguard sovereignty over the nation's territories."
Beijing has also warned Tokyo that trade could be hurt by the flare-up in tension, while the United States called for ''cooler heads to prevail.''
The purchase sparked anti-Japanese protests in several Chinese cities, and the Japanese Consulate in Shanghai reported physical attacks on its nationals.
Fujimura called on China to secure the safety of Japanese nationals in China.
"We are asking our Foreign Ministry to step up measures to ensure the safety of Japanese residents in China, distributing information about demonstrations and instructing Japanese citizens to be on the alert during their stay in China," he said.
The uninhabited islands, which are also claimed by Taiwan, are located near rich fishing grounds and potentially huge maritime natural-gas fields.
With reporting by AFP, Reuters, and AP