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.
During his nomination hearing, Mr Tillerson said the US should block access to islands being built by China in the South China Sea, likening it to Russia’s annexation of Crimea from Ukraine.
“We’re going to have to send China a clear signal that first, the island-building stops and second, your access to those islands also is not going to be allowed,” he told the US Senate Foreign Relations Committee.
The Chinese state media responded by warning that such actions would lead to a “devastating confrontation”.
Rival countries have wrangled over territory in the South China Sea for centuries, but tension has steadily increased in recent years.
Its islets and waters are claimed in part or in whole by Taiwan, China, Vietnam, the Philippines, Malaysia and Brunei.
Beijing has been building artificial islands on reefs and carrying out naval patrols in waters also claimed by these other nations.
Although the Obama administration insisted it was neutral, it spoke out strongly against the island-building and sought to build ties with, and among, the South East Asian nations whose claims overlap those of China.
In July an international tribunal ruled against Chinese claims, backing a case brought by the Philippines, but Beijing said it would not respect the verdict.
The frictions have sparked concern that the area is becoming a flashpoint with global consequences.