The world’s second-largest economy claims 90 percent of the strategic waterway, home to over 250 islands and rich natural gas reserves, but Vietnam, Brunei, Malaysia, the Philippines and Taiwan also assert ownership of certain parts. And to the anger of other claimants, Beijing has been ramping up construction in the area— reports recently emerged of Chinese rocket launchers in the Spratly Islands.

Trump vowed to halt Beijing’s construction of artificial islands days after taking office and despite the president’s recent niceties with Chinese President Xi Jinping, the issue remains a sticking point the bilateral relationship. Last week, the Chinese navy deployed two missile frigates to a U.S. Navy warship that sailed near the China-claimed Spratly Islands.

“We oppose countries militarizing artificial islands and enforcing excessive maritime claims,” Mattis stated. “We cannot and will not accept unilateral, coercive changes to the status quo.”

Mattis also said he expected “friction” in the U.S.-China relationship, adding that “while competition between the U.S. and China is bound to occur, conflict is not inevitable.”