A United States aircraft carrier strike group has begun patrols in the South China Sea amid growing tension with Beijing over control of the disputed waterway.
China's Foreign Ministry has warned Washington against challenging Beijing's sovereignty in the area.
The US Navy said the force, including Nimitz-class aircraft carrier USS Carl Vinson, began routine operations in the South China Sea on Saturday.
The announcement was posted on the Vinson's Facebook page.
The strike group's commander, Rear Admiral James Kilby, said that weeks of training in the Pacific had improved the group's effectiveness and readiness.
"We are looking forward to demonstrating those capabilities while building upon existing strong relationships with our allies, partners and friends in the Indo-Asia-Pacific region," he was quoted as saying by the Navy News Service.
Friction between the United States and China over trade and territory under US President Donald Trump have increased concerns that the South China Sea could become a flashpoint.
China wrapped up its own naval exercises in the South China Sea on Friday.
War games involving China's aircraft carrier have unnerved neighbours with whom it has long-running territorial disputes.
Beijing lays claim to almost all of the resource-rich South China Sea, through which about $5 trillion worth of trade passes each year.
Brunei, Malaysia, the Philippines, Taiwan and Vietnam also claim parts of the waters that command strategic sea lanes and have rich fishing grounds, along with oil and gas deposits.
The United States has criticised Beijing's construction of man-made islands and build-up of military facilities in the sea, and expressed concern they could be used to restrict free movement.