If China attacks the Philippines in an effort to control a vital international shipping lane, the United States will help defend its ally, Secretary of State Mike Pompeo warned Friday.

“China’s island-building and military activities in the South China Sea threaten your sovereignty, security, and therefore economic livelihood, as well as that of the United States,” Pompeo said at a press conference in the Philippine capital of Manila. “[A]ny armed attack on Philippine forces, aircraft, or public vessels in the South China Sea will trigger mutual defense obligations under Article 4 of our Mutual Defense Treaty.”