Amid escalating tensions with the Philippines, China on Monday called South China Sea arbitration “a pure political drama.”

“South China Sea arbitration was a pure political drama staged in the name of the law with the US pulling strings behind the scenes,” said China’s Foreign Ministry in a statement.

The sharp reaction came after the Philippines Coast Guard released photos of China’s Coast Guard using water cannons against one of its vessels on a supply mission to Ayungin Shoal in the South China Sea, a spot also claimed by China under the name Renai Reef.

Referring to the United Nations Convention on the Law of the Sea (UNCLOS), the Philippines’ President Ferdinand Marcos Jr. said Manila has lodged a protest with China over the firing of water cannons at its vessels.

Washington backed Manila over the issue and said: "The United States reaffirms an armed attack on Philippine public vessels, aircraft, and armed forces — including those of its Coast Guard in the South China Sea – would invoke the 1951 US Philippines Mutual Defense Treaty."

The territorial claims in the disputed South China Sea remain highly contested, despite repeated calls for a negotiated settlement and avoiding breaches of sovereignty.

In 2016, however, China was dealt a blow when the Permanent Court of Arbitration (PCA), an international tribunal based in The Hague, ruled that its nine-dash line claim has no legal basis under international law.

"China firmly opposes the US statement attacking China’s maritime rights in the South China Sea and supporting the Philippines' violation of China's sovereignty,” the ministry said.

Beijing asked the US "to stop using the South China Sea issue to sow discord and respect China's territorial and maritime rights," the statement added.

The PCA also said that China has violated the sovereign rights of the Philippines, which had filed the case.

This was the first time an international tribunal had ruled on the legality of China’s claims, and the verdict had potential implications for other countries involved in the disputes.

“The so-called award (PCA decision) contravenes international laws, including UNCLOS, and is illegal, null and void,” Beijing said in a sharp rebuke to the US.

“The US’ attempt to make an issue of the illegal award will not affect China’s firm resolve to safeguard its territorial sovereignty, maritime rights and interests in accordance with the law,” said the ministry.

Philippines lodges protest

Philippines Foreign Secretary Enrique Manalo met with Chinese Ambassador Huang Xilian to lodge a protest over the incident, said President Ferdinand Marcos Jr.

“We continue to assert our sovereignty … we continue to assert our territorial rights in the face of all of these challenges and consistent with the international laws and UNCLOS,” Marcos told reporters, referring to the United Nations Convention on the Law of the Sea

Marcos was also expected to chair a command conference meeting of the Philippines armed forces to discuss the next steps.

But Marcos added that the Philippines will “still have to keep communicating with the Chinese government, with President Xi (Jinping), with Beijing. We still have to keep communicating with them because we need to really come to a conclusion.”

On Monday the Chinese Coast Guard said that the vessels it fired on had “disregarded” its “repeated dissuasion and warnings.”

The Philippines sent two ships to “intrude” into the waters of the reef, violating the Declaration on the Conduct of Parties in the South China Sea, and “attempting to transport construction materials for maintenance and reinforcement to the illegally grounded military vessel. Chinese coast guard ships lawfully intercepted and took warning enforcement measures,” it said.

“After multiple warnings were ineffective, water cannons were used as a warning to avoid direct confrontation. The on-site operation was professional and restrained, and there is no room for criticism,” it added.

Marcos said it was a “good thing no one was injured. But the supplies that reached the BRP Sierra Madre weren't enough so that's what we need to discuss on what to do next.”

Manila summoned Ambassador Huang “to strongly protest the actions” of the Chinese Coast Guard.

“In 1999, the Philippines sent a military vessel and deliberately ran it aground at Renai Jiao, attempting to change the status quo of Renai Jiao illegally,” Beijing claimed.

In response, Beijing said China “made serious démarches to the Philippines, demanding the removal of the vessel.”

“The Philippines promised several times to tow it away but has yet to act. Not only that, the Philippines sought to overhaul and reinforce the military vessel in order to permanently occupy Renai Jiaom” it added.

The ministry said the Philippines sent two vessels that “intruded” into the adjacent waters of Renai Jiao last weekend and “tried to deliver the construction materials for overhauling and reinforcing the grounded military vessel.”

“Such actions violated China’s sovereignty and the Declaration on the Conduct of Parties in the South China Sea,” it added.