The stage is set for significantly heightened tensions along China’s southern coast. China recently warned that Indian companies should not engage in oil exploration in Vietnamese waters claimed by China, and India rebuffed China’s implicit claim that those waters are Chinese territories.

The Chinese message referred to portions of the South China Sea that are in dispute between China and Vietnam, with a Chinese foreign ministry spokesperson stating, “Our consistent position is that we are opposed to any country engaging in oil and gas exploration and development activities in waters under China’s jurisdiction.”

While many of the disputes in the region refer to the Paracel and Spratly islands (both of which are much farther to the east), China has argued that virtually the entire South China Sea—in some official presentations extending even into Indonesian waters—are in fact Chinese territory. The latest Chinese action is in support of this very expansive view.

The Indian company ONGC Videsh (a subsidiary of an Indian state-owned oil and gas company) is not the first multinational company to be warned by the Chinese about Vietnamese-sanctioned commercial operations off the Vietnamese coast. BP and ExxonMobil are perhaps the most famous targets of China’s ire in this regard. What makes the incident with India particularly worrisome is that it follows directly on the heels of an incident off the same coast reportedly involving an Indian naval vessel. When the Indian amphibious warship INS Airavat was departing Nha Trang for Haiphong in late July—and still in Vietnam’s exclusive economic zone—it was reportedly contacted on an open radio channel by a caller claiming to be the “Chinese navy.” The caller stated that the Indian vessel was entering Chinese waters and demanded that it identify itself and state its purpose.

Although India eventually issued a statement defending its maritime rights, all the parties (China, India, and Vietnam) have sought to downplay the July incident. This new Chinese declaration, however, would suggest that there is a move afoot in Beijing to firmly assert its claims against all comers. The South China Sea is likely to become tenser in the coming months.