Philippine President Ferdinand Marcos Jr. says a mutual defense treaty with the United States will be invoked if any Filipino service member is killed in an attack by a foreign power. The remarks come amid China's increasingly coercive actions in the South China Sea.

Marcos spoke to reporters in Manila on Monday upon his return from the US, where he met the leaders of Japan and the US for their first trilateral summit.

Marcos said the Philippines will ramp up security cooperation with its allies and partners. He said the country in recent years has been subjected to illegal, aggressive and irresponsible actions in the South China Sea.

Marcos said the Mutual Defense Treaty with the US will be invoked if a Filipino soldier is killed on an attack by any foreign power -- suggesting he expects a US military response even if such an attack comes from non-military entities, such as coast guards and private-sector vessels.

Marcos also referred to an agreement that a past Philippine administration is said to have struck with China. It was believed to be about maintaining the status quo between the countries.

Marcos said his team is trying to verify if there was such an agreement. He denounced it as a "secret" pact, since this has not been made public.

Marcos pledged that, if there really were such an agreement, he would revoke it.