The United States warned on Monday that any armed attack on Philippine armed forces and Philippine Coast Guard civilian vessels could trigger its 72-year-old Mutual Defense Treaty with the Philippines.

"The United States reaffirms that Article IV of the 1951 US-Philippines Mutual Defense Treaty extends to armed attacks on Philippine armed forces, public vessels, and aircraft — including those of its Coast Guard — anywhere in the South China Sea," the US Department of State said in a statement.

The statement came a day after a China Coast Guard ship collided with a Philippine resupply vessel that was conducting a routine resupply mission at Ayungin Shoal.

During the same resupply mission, a Chinese maritime militia vessel also bumped the BRP Cabra, one of the PCG ships that were escorting the boat contracted by the AFP.

Defense Secretary Gilbert Teodoro Jr. said the collisions, which President Ferdinand Marcos Jr. ordered investigated by the PCG, were "intentional."

For its part, the Philippines' Department of Foreign Affairs said further study is needed to invoke the MDT due to the incident.

"The matter of filing another case is something that is still being studied by the government. But, of course, all incidents like these will actually bolster the case that it is not the Philippines that is the aggressor but the other party, which is China," DFA spokesperson Teresita Daza said.

"It's a provocation on the side of China but at this point, whether it will constitute an armed attack that would allow the invoking of the MDT is something that needs to be studied," she added.

The Philippines and the US agreed under the MDT, signed in 1951, that an armed attack on either of the parties would be considered an attack on the other, necessitating common or joint actions.

Ambassador summoned

The Philippines on Monday summoned China's Ambassador to the Philippines, Huang Xilian, to explain the "blocking maneuvers" conducted by its vessels.

China has insisted on its territorial rights over the Ayungin Shoal, also known as Second Thomas Shoal, which is in the Philippines' exclusive economic zone.

Swarms of CCG ships and Chinese maritime militia vessels had been monitored in the WPS, which Philippine officials said was a blatant disrespect of the 2016 Arbitral Award on the South China Sea.

In July 2016, the Philippines won the arbitral case it lodged against China in the Permanent Court of Arbitration.

The US said it stands with the Philippines in the face of China's "dangerous and unlawful actions obstructing" a 22 October resupply mission to Second Thomas Shoal in the South China Sea.

"By conducting dangerous maneuvers that caused collisions with Philippine resupply and Coast Guard ships, the PRC Coast Guard and maritime militia violated international law by intentionally interfering with the Philippine vessels' exercise of high seas freedom of navigation," the US State Department said.

"The PRC's (People's Republic of China) conduct jeopardized the Filipino crew members' safety and impeded critically needed supplies from reaching service members stationed on the BRP Sierra Madre. Obstructing supply lines to this longstanding outpost and interfering with lawful Philippine maritime operations undermines regional stability," it added.

Citing the 2016 arbitral ruling on the South China Sea, the US noted that China's territorial claims to Ayungin Shoal, a low-tide elevation outside the territorial sea of another high tide feature, were unfounded.

Unsafe moves

"The unsafe maneuvers on 22 October and the PRC water cannoning of a Philippine vessel on 5 August are the latest examples of provocative PRC measures in the South China Sea to enforce its expansive and unlawful maritime claims, reflecting disregard for other states lawfully operating in the region," the State Department stressed.

Aside from the US, Japan, France, Canada, Australia and Germany also condemned China's aggressive actions in the West Philippine Sea.

In a separate statement, Canada condemned what it described as "unlawful and dangerous conduct" of the People's Republic of China in the WPS, which "provoked two collisions with Philippine vessels engaged in routine operations inside the Philippine exclusive economic zone, in the vicinity of Ayungin Shoal."

"The PRC's actions are unjustified. China has no lawful claim to the West Philippine Sea. Its actions are incompatible with the obligations of a signatory to the UN Convention on the Law of the Sea," the Canadian Embassy in Manila said.

"Continuing acts of intimidation and coercion undermine safety, stability, and security across the region, and increase the risk of miscalculation," it added.

France, Japan, Germany, too

France expressed its "deep concern" over the dangerous maneuvers of the CCG against Philippine vessels "engaged in the Philippine exclusive economic zone."

"France calls for respect of the freedom of navigation guaranteed by international law and recalls its attachment to the United Nations Convention on the Law of the Sea and the decision rendered by the Arbitral Court on 12 July 2016," the Embassy of France to the Philippines said.

Japan, likewise, stressed that it was "seriously concerned" and "alarmed" about the collision of Chinese vessels with Philippine vessels.

"Seriously concerned about and alarmed by the collision between Chinese and Philippines vessels," Japanese Ambassador to the Philippines Koshikawa Kazuhiko said in a tweet.

Koshikawa said Japan "strongly opposes any unilateral attempts to change the status quo by force or coercion."

"Germany is very concerned about recent confrontations in the SCS involving Chinese coast guard ships and maritime militia vessels in the exclusive economic zone of the Philippines," German Ambassador to the Philippines Andreas Pfaffernoschke said in a separate tweet.