A U.S. Navy warship patrolled near at least two Chinese-held outposts in the disputed Spratly island chain in the South China Sea on Sunday, challenging Beijing’s maritime claims amid growing tensions between the two countries.

The guided-missile destroyer USS Decatur navigated past Gaven and Johnson reefs over the course of a 10-hour patrol in the South China Sea, sailing within 12 nautical miles of both features in what amounts to a freedom of navigation operation, or FONOP.

The two outposts are among seven where China has built heavily fortified artificial islands since 2013, raising fears among its Asian neighbors and in the U.S. that Beijing could use them to enforce its claims to almost all of the South China Sea.

U.S. forces operating in the region navigate through international waters regularly and always abide by international law, a U.S. official said, adding that such patrols demonstrate the U.S. will “fly, sail and operate wherever international law allows.”

“That is true in the South China Sea as in other places around the globe,” the official said. “FONOPs are not about any one country, nor are they about making political statements,” the official said.