Deputy Secretary of State Kurt Campbell "raised serious concerns" about Chinese actions in a call with Executive Vice Foreign Minister Ma Zhaoxu, the State Department said.

Campbell "reiterated that US commitments to the Philippines under the Mutual Defense Treaty remain ironclad," State Department spokesman Matthew Miller said in a statement.

Campbell also called for "peace and stability" in the Taiwan Strait, in the wake of China's military drills around the self-governing democracy following the inauguration of President Lai Ching-te, and renewed US concerns about Chinese exports that support Russia's defense industry.

Chinese coast guard personnel wielding knives, sticks and an ax surrounded and boarded three Filipino navy boats last week, in the most serious of a number of escalating confrontations.

China has been asserting claims in the strategic South China Sea and separately has put pressure on Taiwan, which it considers part of its territory awaiting reunification.

The United States provides weapons to Taiwan but has been deliberately ambiguous on whether it would come to its defense in a Chinese invasion.

By contrast, the United States has a defense treaty with the Philippines that dates to 1951 that says Washington will come to its former colony's aid in case of an "armed attack."

The United States has repeatedly stressed its commitments to the treaty without spelling out publicly at which point China has crossed a line.

Despite multiple areas of tensions, President Joe Biden's administration has worked to expand communication with China to reduce the chance of greater conflict.

Campbell's call was "part of ongoing efforts to maintain open lines of communication" between the two powers and "responsibly manage competition in the relationship," Miller said.