The US Navy sent a guided-missile destroyer within 12 nautical miles of artificial islands built by China in the South China Sea on Tuesday, a US defence official said, in a challenge to China's territorial claims in the area.
The official said the USS Lassen was sailing near Subi and Mischief reefs in the Spratly archipelago, features that were submerged at high tide before China began a massive dredging project to turn them into islands in 2014.
"The operation has begun ... It will be complete within a few hours," said the official, who spoke on condition of anonymity.
The mission would be the start of a series of challenges to China's territorial claims in one of the world's busiest sea lanes, another US defence official said.
The second official earlier said the ship would likely be accompanied by a US Navy P-8A surveillance plane and possibly P-3 surveillance plane, which have been conducting regular surveillance missions in the region.
The patrols represent the most serious US challenge yet to 12-nautical-mile territorial limits China claims around the islands and are certain to anger Beijing, which said last month it would "never allow any country" to violate its territorial waters and airspace in the Spratlys.
China's Foreign Minister Wang Yi said Beijing was trying to verify if the US ship had entered the 12-mile zone.
"If true, we advise the US to think again and before acting, not act blindly or make trouble out of nothing," the Foreign Ministry quoted Wang as saying.
The Chinese Embassy in Washington said the concept of freedom of navigation should not be used as an excuse for muscle flexing and the United States should "refrain from saying or doing anything provocative and act responsibly in maintaining regional peace and stability."
Additional patrols would follow in the coming weeks and could also be conducted around features that Vietnam and the Philippines have built up in the Spratlys, the second US official said.
"This is something that will be a regular occurrence, not a one-off event," said the official. "It's not something that's unique to China."
White House spokesman Josh Earnest referred questions on any specific operations to the Pentagon but said the United States had made clear to China the importance of free flow of commerce in the South China Sea.
"There are billions of dollars of commerce that float through that region of the world," Earnest told a news briefing. "Ensuring that free flow of commerce ... is critical to the global economy," he said.
The patrols were the first within 12 miles of the features since China began building the reefs up at the end of 2013. The United States last went within 12 miles of Chinese-claimed territory in the Spratlys in 2012.
The decision to go ahead follows months of deliberation and risks significantly upsetting already strained ties with China, the world's second-biggest economy, with which US business and economic interests are deeply intertwined.
US Congressman Randy Forbes, chairman of the House Armed Services Seapower and Projection Forces Subcommittee, earlier praised the plan.
"The passage of US vessels within 12 nautical miles of China's man-made features in the South China Sea is a necessary and overdue response to China's destabilising behavior in the region," Forbes said.
China claims most of the South China Sea, one of the world's busiest sea lanes, through which more than $5 trillion of world trade passes every year. Vietnam, Malaysia, Brunei, the Philippines and Taiwan have rival claims.
Under the UN Convention on the Law of the Sea, 12-nautical mile limits cannot be set around man-made islands built on previously submerged reefs.
Washington worries that China has built up the islands with the aim of extending its military reach in the South China Sea. China says they will have mainly civilian uses as well as undefined defence purposes.
The patrols come just weeks ahead of a series of Asia-Pacific summits President Barack Obama and Chinese President Xi Jinping are expected to attend in the second half of November.
Xi surprised US officials after a meeting with Obama in Washington last month by saying that China had "no intention to militarise" the islands.
Even before that, however, satellite photographs had shown the construction of three military-length airstrips by China in the Spratlys, including one each on Mischief and Subi reefs.
Some US officials have said that the plan for patrols was aimed in part at testing Xi's statement on militarisation.
In May, the Chinese navy issued eight warnings to the crew of a US P8-A Poseidon surveillance aircraft that flew near the artificial islands but not within the 12-mile limit, reported CNN, which was aboard the US aircraft.
In 2013, Obama ordered two B-52 bombers to fly through an Air Defense Identification Zone that China established in the East China Sea over territory contested with Japan.
Pentagon officials say the United States regularly conducts freedom-of-navigation operations around the world to challenge excessive maritime claims.
In early September, China sent naval vessels within 12 miles of the Aleutian Islands off Alaska. China said they were there as part of a routine drill following exercises with Russia.