US aircraft carrier the USS Carl Vinson has started what it calls "routine operations" in the South China Sea, with a fleet of supporting warships, BBC reports.
The deployment comes days after China's foreign ministry warned Washington against challenging Beijing's sovereignty in the region.
China claims several contested shoals, islets and reefs in the area.
It has been constructing artificial islands with airstrips in the South China Sea for a number of years.
The aircraft carrier was last in the South China Sea two years ago, for exercises with Malaysia's navy and air force and has made 16 voyages to the region in its 35 years of US navy service.
Shortly after US President Donald Trump took office on 20 January, his administration vowed to prevent China from taking territory in the South China Sea.
On Wednesday, Chinese foreign ministry spokesman Geng Shuang said: "We urge the US not to take any actions that challenge China's sovereignty and security."