(CNN) - China has test-fired at least one anti-ship missile in recent days in the area around the contested Spratly Islands in the South China Sea, the United States military confirmed Tuesday.
"The Pentagon was aware of the Chinese missile launch from the man-made structures in the South China Sea near the Spratly Islands," spokesman Lt. Col. Dave Eastburn said.
In a notice to mariners released on Saturday, China said it would be conducting military exercises until Wednesday in a large area of the South China Sea, north of the Spratly Islands. The missile test was part of those exercises, according to NBC News, which first reported the Chinese action.
More tests were expected, NBC said, citing unnamed US officials.
CNN has asked Chinese authorities for comment on the report.
China has claimed almost the entire 1.3 million square mile South China Sea as its sovereign territory, including the area of the Spratlys, which are also claimed in part by the Philippines, Vietnam and Taiwan.
To reinforce its claim, China has created and fortified artificial islands in the chain in the southern part of the South China Sea, despite a pledge made by President Xi Jinping during a White House visit in 2015 that the islands would not become military outposts.
"What's truly disturbing about this act is that it's in direct contradiction to President Xi's statement ... that he would not militarize those man-made outposts," Eastburn said Tuesday.
"I'm not going to speak on behalf of all the sovereign nations in the region, but I'm sure they agree that (China's) behavior is contrary to its claim to want to bring peace to the region and obviously actions like this are coercive acts meant to intimidate other (South China Sea) claimants," Eastburn added.
But it's not just China that is flexing military muscles in the South China Sea. The US and its allies and partners are beefing up their presence in the region.
In June, one of the US Navy's largest warships, the aircraft carrier USS Ronald Reagan, partnered with one of Japan's largest naval vessels, the helicopter destroyer JS Izumo, for exercises in the South China Sea.
At the weekend, the US Navy announced the littoral combat ship USS Montgomery was making a port call in Davao, Philippines. The shallow-draft ship, designed for action near shorelines and around islands, is expected to be headed to Singapore as part of a stated US Navy plan to put forward-deploy two ships there this year.
Meanwhile, warships from Australia, Canada and France have been in and around the South China Sea during the last 12-months, as the US encourages allies to support a free and open Indo-Pacific region.
Tensions have been especially high in the Spratlys in the past month, after a Philippine fishing boat sank following a collision with a Chinese fishing boat on June 9.
The Filipino fishermen claimed they were left to drown by the Chinese boat and were only rescued hours later by a Vietnamese boat. China claims its vessel felt threatened by swarming Philippine boats.
The incident was enough for Manila to register a diplomatic protest with Beijing, though the government of President Rodrigo Duterte so far appears to be reluctant to push Beijing too hard on the issue.
CNN's Ryan Browne contributed to this report.
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