The US warned China on Sunday that an attack on Philippine armed forces in the South China Sea would trigger a 1951 mutual defense treaty between the US and the Philippines.

Secretary of State Antony Blinken made the remark in a written statement commemorating the fifth anniversary of an arbitration tribunal’s decision rejecting China’s extensive territorial claims in the South China Sea.

China, which claims most of the waters within the so-called Nine Dash Line, which is also disputed by Brunei, Malaysia, the Philippines, Taiwan, and Vietnam, restated its rejection of the verdict on Friday.

“The United States reaffirms its July 13, 2020 policy regarding maritime claims in the South China Sea,” Blinken said, referring to the rejection by former President Donald Trump’s administration of China’s claims to offshore resources in most of the South China Sea.

“We also reaffirm that an armed attack on Philippine armed forces, public vessels, or aircraft in the South China Sea would invoke U.S. mutual defense commitments under Article IV of the 1951 U.S.-Philippines Mutual Defense Treaty,” Blinken added.

That article of the treaty says in part that “each Party recognizes that an armed attack in the Pacific area on either of the Parties would be dangerous to its own peace and safety and declares that it would act to meet the common dangers in accordance with its constitutional processes.”

Blinken has made the point before, including during an April 8 conversation with the Philippine foreign minister in which the State Department said he “reaffirmed the applicability” of the treaty to the South China Sea.