Protesters, from the Socialista National Confederation of Labor activist group, display a mock missile during a rally regarding the disputed islands in the South China Sea, in front of the Chinese Consulate in Makati city, metro Manila, July 24, 2015. Reuters

Japan's military leader has warned that all of the South China Sea is under threat from "China’s sphere of military influence” if Beijing uses artificial islands it has built there for military purposes. Meanwhile, the head of the Philippine military, General Hernando Iriberri, told journalists in Manila Thursday that it was investigating reports China had reclaimed three more reefs in the South China Sea, Reuters reported.

Adm. Tomohisa Takei, commander of Japan's Maritime Self-Defense Force, said it is important to ensure the South China Sea remains “free and open waters," the Japan Times reported. “Even though the Indian Ocean and the Western Pacific are geographically separated, they are so closely related that they cannot be separated both politically and economically,” Takei said in a speech Wednesday at a think tank in Washington, D.C. “Therefore, we need to regard security in each of these two oceans as identical.”

China has claimed most of the South China Sea, which is rich in oil and natural gas deposits, despite sovereignty claims by Vietnam, the Philippines, Brunei, Malaysia and Taiwan. The United States and Japan have closely watched the rising tensions over the dispute and warned China not to take further action to claim the South China Sea. To discourage China's potential military threats, the U.S. has also held military drills with regional allies, including the Philippines, in the South China Sea in recent months.

A U.S. government official called on European Union officials Wednesday to denounce China's building and militarization of man-made outposts in the South China Sea. Amy Searight, U.S. deputy assistant secretary of defense for South and Southeast Asia, said Washington needs the EU's help to negotiate for a peaceful resolution.

“It would be helpful if the EU would be a little more clear in terms of backing up these principles,” she told a discussion on U.S. and EU policies toward East Asia at Washington’s Center for Strategic and International Studies, Reuters reported. “A little bit more forward-leaning approach that would support, for example, the idea of a halt to further reclamation, further militarization, would be very useful.”

Meanwhile, China's Defense Ministry slammed the United States Thursday for "militarizing" the South China Sea by staging patrols and joint military drills there. "China is extremely concerned at the United States' pushing of the militarization of the South China Sea region," said Defense Ministry spokesman Yang Yujun during a news briefing. "What they are doing can't help but make people wonder whether they want nothing better than chaos."