China has warned a US Navy guided-missile destroyer to keep away from a group of man-made islands it has built in the disputed South China Sea.
China says it followed and challenged USS Lassen as it sailed within 12 nautical miles of Subi Reef around the islands in the Spratly archipelago.
China claims it owns the waters in some of the world's busiest sea lanes - other nations challenge it.
China's Foreign Ministry said the "relevant authorities" had monitored, followed and warned the USS Lassen as it "illegally" entered waters near islands and reefs in the Spratlys.
"China will resolutely respond to any country's deliberate provocations," it said in a statement.
"We will continue to closely monitor the relevant seas and airspace, and take all necessary steps in accordance with the need.
"China strongly urges the US side to conscientiously handle China's serious representations, immediately correct its mistake and not take any dangerous or provocative acts that threaten China's sovereignty and security interests," it said.
A US defence official confirmed the ship had sailed within the disputed area and a second defence official said the mission, which lasted a few hours, also included Mischief Reef.
He said it would be the first in a series of freedom-of-navigation exercises aimed at testing China's territorial claims.
The US insists there will be more patrols in the near future and could also take in areas that Vietnam and the Philippines have built up interests in.
"This is something that will be a regular occurrence, not a one-off event," said the second US official. "It's not something that's unique to China."
A White House official has said the US has made it clear to China how important it considers the free flow of commerce in the South China Sea to be.
The US was last within the zone in 2012 - it is the first time it has been within 12 miles of the seven Chinese outposts since Beijing began building the reefs up at the end of 2013.
China claims most of the South China Sea, through which more than $5trn (around €4.5trn) of world trade passes every year.
Both Subi and Mischief Reefs were submerged at high tide before China began a huge dredging project to turn them into islands in 2014.
Under UN law, 12-nautical mile limits cannot be set around man-made islands built on previously submerged reefs.
Vietnam, Malaysia, Brunei, the Philippines and Taiwan have rival claims.
