A day after the Pentagon announced its intentions to send more warships through the area around some newly created islands in the South China Sea, China is warning they will take measures to defend their claims over those islands.

A US warship, the USS Fort Worth, sailed through the vicinity of the islands earlier this week without incident, but that was before the Pentagon made clear they intend to make such patrols a direct challenge to Chinese maritime holdings.

The Pentagon says the efforts are meant to “ensure freedom of navigation,” and they say that the manmade islands in the Spratly chain are not to be considered extensions of Chinese territory.

The Spratlys are partially held by China, Taiwan, Malaysia, the Philippines, and Vietnam. The Sultanate of Brunei also has claims in the islands. The US traditionally affirms the claims of every single nation, except China, where they overlap.

The islands themselves are of little value, though the region is frequently used for trade, and the US seems to view Chinese claims as a threat to commerce, despite China so far having shown no sign of preventing civilian travel through the area.