MANILA, Philippines (AP) — Two Philippine supply boats breached a Chinese coast guard blockade in the South China Sea on Wednesday in a recurring confrontation near a disputed shoal some fear could spark a larger security crisis that could draw in the United States.

Two Philippine coast guard ships escorted the smaller supply boats, but it wasn’t immediately clear whether the ships were blocked by the Chinese coast guard from coming closer to the Second Thomas Shoal, where a small contingent of Filipino marines has stood guard for years aboard a long-marooned but still actively commissioned warship, the BRP Sierra Madre.

China also claims the shoal and has surrounded it with its coast guard ships and militia vessels to prevent the Philippines from delivering construction materials that Beijing fears could be used to reinforce the Sierra Madre and turn it into a permanent territorial outpost.

“Despite attempts by a significant number of China coast guard and Chinese maritime militia vessels to block, harass, and interfere with the routine rotation and resupply mission,” the two Philippine boats managed to deliver provisions to the Filipino forces at the shoal, a Philippine government body overseeing the disputed waters said in a statement late Wednesday.

“The Philippines’ resupply missions and maintenance of BRP Sierra Madre are part of regular operations in line with domestic and international law and ensures safety and well-being of our stationed personnel,” the inter-agency body said.

It didn’t provide other details of the Chinese coast guard’s actions, which it has condemned in the past as dangerous maneuvers that have nearly caused collisions. and violated international safety regulations at sea.

The dangerous confrontation on Wednesday is the latest flare-up from the long-simmering territorial disputes in the South China Sea, one of the world’s busiest trade routes. The conflicts, which involve China, the Philippines, Vietnam, Malaysia, Taiwan and Brunei, are regarded as a potential Asian flashpoint and have also become a delicate fault line in the U.S.-China rivalry in the region.

In early August, a Chinese coast guard ship used a water cannon against one of two Philippine supply boats to prevent it from approaching the Second Thomas Shoal. The brazen move, which was caught on video, outraged President Ferdinand Marcos Jr. and prompted the Department of Foreign Affairs in Manila to summon the Chinese ambassador to hand a strongly worded protest.

Washington reacted by renewing a warning that it’s obligated to defend the Philippines, its oldest treaty ally in Asia, if Filipino forces, aircraft and vessels come under an armed attack, including in the South China Sea.

The Chinese Foreign Ministry accused Washington then of “threatening China” by raising the possibility of the U.S.-Philippines mutual defense treaty being activated. Beijing has repeatedly warned the U.S. not to meddle in the territorial disputes.

Later in August, the Philippines again deployed two boats, which succeeded in maneuvering past the Chinese coast guard blockade and completing the delivery of supplies to the Filipino forces at the Second Thomas shoal. Two Philippine coast guard ships securing the supply boats, however, were blocked and prevented by Chinese coast guard ships from maneuvering closer to the shoal. A U.S. Navy surveillance aircraft flew in circles in support of the Philippine vessels as the standoff lasted for more than three hours.

Philippine Defense Secretary Gilberto Teodoro expressed concern over China’s dangerous actions at sea and said the government was ready to respond to potential emergencies, including a possible collision of Chinese and Philippine ships in the disputed waters.

“Naturally the concern is always there, and we take that into account,” Teodoro said in response to a reporter’s question Tuesday night. “We have plans depending on what happens.”

The Philippine coast guard invited a small group of journalists, including two from The Associated Press, in August to join its ships that secured the supply boats as part of a new strategy aimed at exposing China’s increasingly aggressive actions in the South China Sea.

A 2016 arbitration ruling set up under the 1982 U.N. Convention on the Law of the Sea invalidated Beijing’s claims on historical grounds to virtually the entire South China Sea. But China refused to participate in the arbitration sought by the Philippines, rejected the decision as a sham and continues to defy it.