China has asserted its “indisputable sovereignty” over parts of the South China Sea after the Trump administration vowed to prevent China from taking territory in the region.
The Chinese foreign ministry said Beijing would “remain firm to defend its rights in the region”.
White House spokesman Sean Spicer said on Monday the US would “make sure we protect our interests there”.
Barack Obama’s administration refused to take sides in the dispute.
It did, however, send B-52 bombers and a naval destroyer last year, and the then US Secretary of State John Kerry spoke out over what he called “an increase of militarisation from one kind or another” in the region.
Several nations claim territory in the resource-rich South China Sea, which is also an important shipping route.
The new US president has taken a tough stance against China, and Mr Spicer told reporters “the US is going to make sure we protect our interests” in the South China Sea.
“If those islands are, in fact, in international waters and not part of China proper, yeah, we’ll make sure we defend international interests from being taken over by another country,” he said, without giving further details.
The Chinese government responded by saying that the US was “not a party to the South China Sea issue”.
Foreign Ministry spokesperson Hua Chunying said China was “committed to peaceful negotiations with all countries concerned” in the dispute, and said it “respects the principles of freedom of navigation and over-flight in international waters”.
But, she went on: “Our position is clear. Our actions have been lawful.”
Mr Spicer’s comments echo those of Donald Trump’s new Secretary of State Rex Tillerson.