The US Navy has started sailing its warships within 12nm miles of the artificial islands built by China in the disputed South China Sea.
The US earlier said they will send warships in the disputed area to contest Chinese territorial claims to the artificial islands, where China has reportedly begun military construction.
According to US officials, the navy's Burke-class guided-missile destroyer USS Lassen performed the patrol, sailing near the Subi and Mischief reefs in the Spratly archipelago.
An official was quoted by Reuters as saying: "The operation has begun. It will be complete within a few hours."
In late 2013, China started construction work on reefs that have been submerged, turning them into islands.
Recent Airbus Defence and Space imagery revealed the presence of helipads and runways on the island, proving that China is building outposts far from its shores.
According to International maritime law, countries can claim ownership of the 12nm area surrounding natural islands, but not the ownership of submerged features that have been raised by human intervention, BBC News reported.
US Defense Secretary Ash Carter earlier said that "the US will fly, sail and operate wherever international law allows, as we do around the world. And the South China Sea is not and will not be an exception."
Meanwhile, China has warned that the USS Lassen has "illegally" entered waters near islands and reefs, the news agency reported.
A statement from the China Foreign Ministry read as: "China will resolutely respond to any country's deliberate provocations.
"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."
Several countries, including Vietnam, Malaysia, Brunei, the Philippines, and Taiwan also have claims over the disputed islands.
Image: The US Navy's Burke-class guided-missile destroyer USS Lassen sailed within 12nm miles of the artificial islands built by China. Photo: courtesy of U.S. Navy photo by Photographer's Mate2nd Class Inez Lawson.