Philippine President Ferdinand Marcos Jr. has vowed to step up the country’s defense of its maritime zones in the South China Sea after Filipino and Chinese vessels collided over the weekend in what Manila has labeled a “serious escalation.”

Both countries said Monday they have lodged mutual diplomatic protests, while Manila summoned the Chinese ambassador.

On Sunday, the Philippines accused China of causing “severe damage” to one of its vessels, which was part of a convoy on a resupply mission to Second Thomas Shoal, a submerged reef in the Spratly Islands in the South China Sea. Filipino troops are stationed at its outpost on the BRP Sierra Madre, a warship grounded at the shoal in 1999 to buttress Manila’s maritime claims.

“We remain undeterred,” Marcos said Sunday in a post on X, formerly Twitter.

“The aggression and provocations perpetrated by the China Coast Guard and their Chinese Maritime Militia against our vessels and personnel over the weekend have only further steeled our determination to defend and protect our nation’s sovereignty, sovereign rights, and jurisdiction in the West Philippine Sea.”