On her third day in an Asian ally that is essential to the United States’ efforts to offset China’s escalating assertiveness in the region, U.S. Vice President Kamala Harris travels to the Philippine island of Palawan in the South China Sea on Tuesday.

Beijing claims nearly the whole South China Sea, which is thought to have enormous oil and gas potential and is used for trade worth billions of dollars annually.

Harris pledged on Monday the United States would defend the Philippines if it came under attack in the waterway, reaffirming Washington’s “unwavering” commitment to its former colony.

Her comments followed a meeting with President Ferdinand Marcos Jr., who welcomed Harris for the first time at the Philippine presidential palace.

Marcos said the two nations’ strong ties had become even more important, given what he called “upheavals” in the region.

Harris’ visit to the Philippines, the highest-level trip to the Philippines by a Biden administration official, is seen as part of Washington’s effort to revive ties with Manila, which moved closer to China under former President Rodrigo Duterte.

“We stand with you in defence of international rules and norms as it relates to the South China Sea,” Harris told Marcos.

A 2016 ruling by an arbitration tribunal in the Hague said Beijing’s South China Sea claims had no legal basis, delivering a victory for Manila.

The Philippines has been unable to enforce the ruling and has since filed hundreds of protests over what it calls encroachment and harassment by China’s coastguard and its vast fishing fleet.

While in Palawan, Harris will also reaffirm Washington’s support for the 2016 tribunal ruling, a U.S. official said.

Palawan is only about 320 km (200 miles) from the Spratly islands, where China has dredged the sea floor to build harbours and airstrips. Parts of the archipelago are also claimed by Brunei, Malaysia, the Philippines, Taiwan and Vietnam.

The vice president will visit a fishing village and tour a Philippine coast guard vessel during her trip, said a U.S. official, on condition of anonymity.

The visit comes with U.S.-Sino tensions high, particularly over Taiwan, the democratically governed island China has long vowed to bring under its control.

“We are not against the U.S.’s interaction with regional countries,” said Chinese foreign ministry spokeswoman Mao Ning when asked to comment about Harris’ visit to Palawan. “But it should be good for regional peace and stability and not damaging to other countries’ interests.”

Illustrating tensions in the area, China denied on Monday that one of its coastguard ships used force to retrieve a piece of a rocket floating in the ocean that was being towed by a Philippine vessel in the South China Sea.

A Philippine military commander said the Chinese coastguard ship “forcefully retrieved” the object by cutting a line attaching it to a Philippine boat.

Dozens of protesters rallied in Manila on Monday against the visit by Harris, saying they did not want the Philippines to be caught between U.S. and Chinese rivalry.

Washington and the Philippines have moved ahead with an Enhanced Defense Cooperation Agreement (EDCA), dating from the Obama administration, though it languished under Duterte.

EDCA allows the United States to maintain a military presence, but not a permanent one, through the rotation of ships and aircraft for humanitarian and maritime security operations at mutually agreed Philippine bases.