MANILA, Philippines (AP) — The Philippine government summoned the Chinese ambassador on Monday to convey a diplomatic protest over the Chinese coast guard’s use of a water cannon against a Filipino supply boat in the disputed South China Sea, a Philippine official said.

The Philippine military on Sunday condemned the Chinese coast guard ship’s “excessive and offensive” use of a water cannon to block a Filipino supply boat from delivering a new batch of troops, food, water and fuel to the Philippine-occupied Second Thomas Shoal in the disputed waters.

The United States, the European Union and their key allies including Australia and Japan expressed support to the Philippines and concern over the Chinese ship’s actions. Washington renewed a warning that it is obliged to defend its longtime treaty ally if Filipino public vessels and forces come under an armed attack including in the South China Sea.

The tense confrontation on Saturday was the latest flare-up in the long-seething territorial conflicts involving China, the Philippines, Vietnam, Malaysia, Taiwan and Brunei.

One of several top Philippine officials dealing with the incident told The Associated Press that the Department of Foreign Affairs in Manila summoned Chinese Ambassador Huang Xilian on Monday morning to convey a strongly worded diplomatic protest.

The protest would underscore how the Chinese coast guard ship’s action violated international regulations aimed at avoiding collisions at sea and the 1982 U.N. Convention on the Law of the Sea, said the official, who spoke on condition of anonymity because of a lack of authority to discuss the government’s actions before they are publicly disclosed.

The disputes in the South China Sea, one of the world’s busiest sea lanes, have long been regarded as an Asian flashpoint and a delicate fault line in the rivalry between the United States and China in the region. China claims ownership over virtually the entire strategic waterway despite international rulings that invalidated Beijing’s vast territorial claims, such as that in 2016 by the Permanent Court of Arbitration, an international body based in The Hague. China rejects that ruling.

Philippine navy personnel on board two chartered supply boats were cruising toward Second Thomas, escorted by Philippine coast guard ships, when a Chinese coast guard ship approached and used a powerful water cannon to block the Filipinos from the shoal that China also claims, the Philippine military and coast guard said Sunday.

The Chinese ship’s action was “in wanton disregard of the safety of the people on board” the Philippine navy-chartered boat and violated international law, the Armed Forces of the Philippines said on Sunday.

The “excessive and offensive actions against Philippine vessels” near the shoal prevented one of the two Filipino boats from unloading supplies needed by Filipino troops guarding the shoal onboard a long-marooned Philippine navy ship, the BRP Sierra Madre, the Philippine military said.

It called on the Chinese coast guard and China’s central military commission “to act with prudence and be responsible in their actions to prevent miscalculations and accidents that will endanger people’s lives.”

The U.S. State Department said in a statement that by “firing water cannons and employing unsafe blocking maneuvers, PRC ships interfered with the Philippines’ lawful exercise of high seas freedom of navigation and jeopardized the safety of the Philippine vessels and crew.” It used the acronym for the People’s Republic of China.

It added that such actions are a direct threat to “regional peace and stability.”

China has long demanded that the Philippines withdraw its small contingent of naval forces and tow away the actively commissioned but crumbling BRP Sierra Madre. The navy ship was deliberately marooned on the shoal in 1999 and now serves as a fragile symbol of Manila’s territorial claim to the atoll.

Chinese ships had blocked and shadowed navy vessels delivering food and other supplies to the Filipino sailors on the ship in the shoal, which Chinese coast guard ships and a swarm of Chinese fishing boats — suspected to be manned by militias — have surrounded for years.

While the U.S. lays no claims to the South China Sea, it has often lashed out at China’s aggressive actions and deployed its warships and fighter jets in patrols and military exercises with regional allies to uphold freedom of navigation and overflight, which it says is in America’s national interest.

China has warned the U.S. to stop meddling in what it calls a purely Asian dispute and has warned of unspecified repercussions.