But that diver was with the Philippine Coast Guard, and the rope was part of a sea barrier placed by Chinese forces to keep Philippine boats away from an area they had a legal right to fish in. In that moment, the Philippines took one of the most forceful steps yet in contesting China's unrelenting territorial claims ever closer to the Philippine Islands.

"The barrier posed a hazard to navigation, a clear violation of international law," the Philippines said in a statement, adding that the action had come on direct orders from President Ferdinand E. Marcos Jr.

Since he took office in June 2022, Mr. Marcos has signaled wanting a more muscular foreign policy approach toward China. But until now, those actions were confined mostly to rhetoric, deepening alliances with the United States and other countries, and releasing videos of aggressive activities undertaken by the Chinese Coast Guard against Philippine vessels.

The surprise this time was that the action was being taken by Manila. It has left little doubt that the Philippines is offering more forceful resistance to China's territorial designs.

While the Biden administration is likely to see that as good news, apprehension is rising in the region about how China might counter that resistance, and whether there could be a risk of sparking a direct military clash among China and the Philippines and its allies, including the United States Navy fleet patrolling the region.

After the rope was cut and the Philippines lifted the anchor that kept it in place, China removed the barrier. On Tuesday, a spokesman for China's foreign ministry brusquely dismissed the Philippine statement. "We advise the Philippines not to cause provocation and cause trouble," he said.