In his first phone call with Philippine Foreign Secretary Manalo, the new US Secretary of State Marco Rubio assured Manila that Washington would "unwaveringly" defend the Philippines in the face of China's actions in the South China Sea.

In a call with Manaro on Wednesday, Rubio criticized China for "dangerous and destabilizing actions in the South China Sea," which the top U.S. diplomat claimed violated international law.

"Secretary Rubio said China's actions undermine regional peace and stability and are inconsistent with international law," the U.S. State Department said in a statement.

"An armed attack against public vessels, aircraft, or armed forces, including the Coast Guard, in the Pacific region, including anywhere in the South China Sea, would trigger a commitment to mutual defense," the U.S. State Department said. ”

The United States and its former colony of the Philippines signed the Mutual Defense Treaty in 1951, which stipulated that the two countries would defend each other in the event of an attack.

Rubio's conference call with the Philippine foreign minister came a day after he held talks with officials at the same level of the Quadrilateral Security Dialogue between India, Japan and Australia.

The Quad diplomatic and security organization, known as the Quadrilateral Security Dialogue, has issued a veiled warning to China that it supports a "free and open Indo-Pacific" region, claiming that "the rule of law, democratic values, sovereignty and territorial integrity are upheld and defended there."

They also said in a statement that "we strongly oppose any unilateral action that seeks to change the status quo by force or coercion".

China has broad claims over much of the South China Sea, but several Southeast Asian countries, including the Philippines, have expressed displeasure.

In a 2016 complaint filed in Manila, the Permanent Court of Arbitration in The Hague ruled that China's territorial claims in the South China Sea lacked legal basis.

But China's refusal to accept the ruling has had little impact on China's increasingly assertive actions in the waters.

Over the past year, standoffs between Chinese and Philippine vessels over disputed islands, waters and reefs have intensified.

On January 14, the Philippines criticized China for deploying "monster ships" in its so-called exclusive economic zone. Philippine National Security Council spokesman Jonathan Malaya said at the time that "this is an escalation and a provocation," adding that the vessel's presence was "illegal" and "unacceptable."

In response, the Philippine Navy conducted so-called "sovereignty patrols" near Scarborough Shoal and conducted live-fire exercises, followed by joint military exercises with the United States.

During that week, the Chinese People's Liberation Army also conducted military readiness exercises in the waters.

Scarborough Shoal is one of the largest coral reefs in the South China Sea. The Philippines claims that the island is within its exclusive economic zone, but actual control of the island is in Chinese hands.