A U.S. warship carried out a “maneuvering drill” when it sailed within 12 nautical miles of an artificial island built up by China in the South China Sea, to show Beijing it was not entitled to a territorial sea around it, U.S. officials said on Thursday.

The operation near Mischief Reef on Thursday, Pacific time, among a string of islets, reefs and shoals over which China has disputes with its neighbors, was the boldest U.S. challenge yet to Chinese island-building in the strategic waterway.

It drew an angry response from China, which President Donald Trump has tried to court in recent weeks to persuade it to take a tougher line on North Korea’s nuclear and missile programs. [nL1N1IQ2FH]

Analysts say previous U.S. “freedom-of-navigation operations” in the Spratly archipelago involved “innocent passage,” in which a warship effectively recognized a territorial sea by crossing it speedily, without stopping.

On Thursday, the destroyer USS Dewey conducted a “man overboard” exercise, specifically to show that its passage within 12 nautical miles was not innocent passage, U.S. officials said.

“USS Dewey engaged in normal operations by conducting a maneuvering drill inside 12 nautical miles of Mischief Reef,” one official said, speaking on condition of anonymity.

“The ship’s actions demonstrated that Mischief Reef is not entitled to its own territorial sea regardless of whether an artificial island has been built on top of it.”

Commander Gary Ross, a Pentagon spokesman, said that freedom of navigation operations are not specific to one country and the Defense Department would release summaries of these operations in an annual report and not sooner.

“We are continuing regular FONOPS, as we have routinely done in the past and will continue to do in the future,” Ross said, using an acronym for freedom of navigation operations.

The Pentagon has not confirmed the most recent operation.

China claims nearly all of the South China Sea and Washington has criticized its construction of islands and build-up of military facilities there, concerned they could be used to restrict free movement and broaden Beijing’s strategic reach.

U.S. allies and partners in the region had grown anxious as the Trump administration held off on carrying out South China Sea operations during its first few months in office.

Greg Poling of Washington’s Center for Strategic and International Studies think tank said that under international law, Mischief Reef was not entitled to a territorial sea as it was underwater at high tide before it was built up by China.

“This was a statement to the Chinese,” he said.

“The previous two freedom-of-navigation operations only challenged China’s demand for prior notification for innocent passage through the territorial sea; this one asserted that there is no territorial sea at all.”

The Trump administration vowed to conduct more robust South China Sea operations after President Barack Obama was criticized for potentially reinforcing China’s claims by sticking to innocent passage.

Even so, this was the first freedom-of-navigation operation since October and since Trump took office in January.

It comes ahead of a visit to Singapore next week by U.S. Defense Secretary Jim Mattis to discuss security with regional counterparts.

Beijing said two Chinese guided-missile warships had warned the U.S. vessel to leave the waters and that it had lodged “stern representations” with the United States.

China’s claims in the South China Sea, through which about $5 trillion in ship-borne trade passes each year, are contested by Brunei, Malaysia, the Philippines, Taiwan and Vietnam.

(Reporting by David Brunnstrom and Idrees Ali in Washington; Editing by James Dalgleish and Marguerita Choy)