A US Navy ship has sailed close to artificial islands built by China in the disputed waters of the South China Sea, US defence officials have said.
Guided-missile destroyer USS Lassen breached the 12-nautical mile zone China claims around Subi and Mischief reefs in the Spratly archipelago.
The freedom of navigation operation represents a serious challenge to China's territorial claims.
China said it was still verifying whether the operation had taken place.
"If true, we advise the US to think again and before acting, not act blindly or make trouble out of nothing," Foreign Minister Wang Yi said.
China claims most of the South and East China seas. Other countries in South East Asia have competing claims for the Spratly Islands, Paracel Islands and Scarborough Shoal, which are thought to have resource-rich waters around them.
The reefs, which were submerged, were turned into islands by China by a massive dredging project which began in late 2013.
China says this work is legal and in a meeting with US President Barack Obama last month in Washington, President Xi Jinping said China had "no intention to militarise" the islands.
But Washington believes Beijing is constructing military facilities, designed to reinforce its disputed claim to most of the region - a major shipping zone.
What is Freedom of Navigation?
The US Freedom of Navigation programme challenges what it deems to be "excessive claims" to the world's oceans and airspace.
It was developed to promote international adherence to the UN Convention on the Law of the Sea, even though the US has not formally ratified the treaty.
In 2013 and 2014, the US conducted Freedom of Navigation operations of different kinds against China, Malaysia, the Philippines, Taiwan, and Vietnam - each of whom occupies territory in the South China Sea.
International maritime law allows countries to claim ownership of the 12-nautical mile area surrounding natural islands but does not allow nations to claim ownership of submerged features that have been raised by human intervention.
A spokesman for the Chinese embassy in Washington earlier said that freedom of navigation "should not be used as excuse to flex muscle and undermine other countries' sovereignty and security".
US Defence Secretary Ash Carter had previously signalled plans for the sail-by, saying the US would "fly, sail and operate wherever international law allows".
A senior US defence official told Reuters news agency the warship began its mission early on Tuesday local time near the reefs and would spend several hours there.
The USS Lassen was expected to be accompanied by a US Navy P-8A surveillance plane and a P-3 surveillance plane, according to the unnamed official, speaking to US media.
Additional patrols could follow in the coming weeks, the official added.
The ship is an Arleigh Burke class missile destroyer, which the US Navy says is among the most powerful destroyers ever built.
It is 155m (509ft) long with a displacement of 9,145 tonnes when fully loaded.
Crewed by a staff of about 330.
It carries two Seahawk helicopters and uses the Aegis defence system.
Weapons include Tomahawk missiles, RUM-139 Asroc anti-submarine missiles and surface-to-air missiles.