Rodrigo Duterte, the firebrand president of the Philippines, on Wednesday called on the US to send warships to defend the island nation against Chinese military aggression as part of a decades-old Mutual Defense Treaty.

A Chinese vessel is accused of ramming a Filipino fishing boat in the Philippines' waters and leaving the fishers for dead.

It's unclear if this attack can be cleanly pinned on China or if the Philippines can demand a US armed response, but the US is under a lot of pressure to flex its muscles, and the Philippines is a key ally that, under Duterte, is increasingly unpredictable.

China routinely ignores international law as it militarizes artificial islands it built in international waters in the South China Sea, and it has frequently threatened US ships.

Rodrigo Duterte, the firebrand president of the Philippines, on Wednesday called on the US to send warships to defend the island nation against Chinese military aggression as part of a decades-old Mutual Defense Treaty.

"I'm calling now, America. I am invoking the RP-US pact, and I would like America to gather their Seventh Fleet in front of China. I'm asking them now," Duterte said, according to CNN Philippines.

Duterte referred to the Seventh Fleet, the part of the US Navy stationed in Japan that patrols and operates in the Pacific and Indian oceans.

His call follows a mid-June incident in which a Chinese ship rammed an anchored Filipino ship, sinking it and leaving 22 on board adrift in the sea. Those fishers left adrift eventually were rescued, but the issue brought forth a bitter argument and a nationwide reckoning with China for the Philippines.

The Filipino ship had been fishing in Recto Bank, about 100 nautical miles outside the island of Palawan and well inside the Philippines' internationally recognized maritime borders. China, however, claims the waters as its own.

The Philippines' aging warships are no match for China's growing naval might, leaving the US's Japan-based fleet as its only resort to stand up to China.

The White House did not immediately respond to a query about whether the US has received a formal request invoking the mutual defense pact from the Philippines.

Even if the treaty was invoked by the Philippines, it's unclear whether the US would consider the fishing boat sinking as an "armed attack" that compels the US to come to its ally's aid.

From cooperation to crisis

The Philippines has already tried legal remedies to China's excessive maritime claim by taking Beijing to the International Court of Arbitration at The Hague, Netherlands. The island nation won its case in 2016, with The Hague ruling that China's claims were excessive.

The Chinese side called this ruling "waste paper" and has since used a quasi maritime militia of fishing boats and sometimes armed vessels to muscle other countries out of the lucrative waterway.

Chinese President Xi Jinping promised former US President Barack Obama in 2015 that he would not militarize the South China Sea, which six nations claim, during a trip to the White House.

But China has steadily built military infrastructure and deployed weapons to the islands. Most recently, it tested anti-ship missiles. China now regularly responds with fury to US Navy ships transiting the international waters of the South China Sea, and military officials have threatened to sink US aircraft carriers multiple times.

Read more: China sets the stage for a 'bloody nose' attack on US aircraft carriers, but it would backfire horribly

Protesters in the Philippines at a rally against China's territorial claims in the Spratly Islands in the South China Sea. Dondi Tawatao/Getty

After some flip-flopping within the Philippines over who to blame and how to handle the ship sinking, Duterte on Wednesday took a harder line than ever against China.

Duterte, throughout his presidency, has sought closer ties with Beijing and at times bashed the US and its military alliance. Previously, Duterte spoke in pragmatic terms, saying that standing up to China on its maritime rights would lead to a war that would destroy his country.

But on Wednesday, Duterte signaled he was ready to fight.

'Okay, let's bomb everything.'

The US military, together with our allies and partners, has the capability to achieve this objective quickly, precisely, decisively, and in ways that control escalation. US Navy

"I will join them," he said of the US Navy, according to CNN Philippines. "I will ride on the boat with admiral of the US, but I will drag along Carpio and the rest of the — Albert. When the Americans say, 'We're here now. Ready?' Ready. I will press the [button]," he said.

Duterte was referring to Senior Associate Justice Antonio Carpio, who has advocated for the Philippines' claims in the South China Sea.

Previously, Duterte had said something similar without invoking the US-Philippines pact.

"When they enter the South China Sea, I will enter. I will ride with the American who goes there first. Then I will tell the Americans, 'Okay, let's bomb everything,'" he said on July 8.

The US Navy's Seventh Fleet typically has roughly 80 ships and submarines, which represent some of the service's best naval assets, but China has spent years modernizing its military to defeat US systems and ships. It's unclear if the Seventh Fleet alone could realistically cow China, which views the issue with a fiery intensity and describes it as a challenge to its sovereignty.

Can the Philippines start a US war with China?

US Navy Chief of Naval Operations Adm. John Richardson recently affirmed that the US would defend the Philippines in the event of an armed attack from China under the 68-year-old Mutual Defense Treaty between Manila, the Philippines' capital, and Washington, but it's unclear if that applies in this case.

China is known to operate a maritime militia of ships that aren't officially part of its military but act aggressively toward foreign ships in an effort to defend China's expansive claims.

China's use of "gray-zone" tactics is meant to inject deniability into conflicts such as the sinking of the Filipino boat. Also, a simple ramming may not qualify as an armed attack.

US officials have warned that the Mutual Defense Treaty could be invoked after an attack by China's maritime militia, with Secretary of State Mike Pompeo saying that "any armed attack on Philippines forces, aircraft or public vessels in the South China Sea" will trigger it.

The US is also under a lot of pressure to stand up to China and reassure its allies in the region, making this case demand some sort of response, even if it's short of war.