Editor's Note: This article originally appeared on Business Insider.

The US Navy tweeted Sunday that it is “not intimidated” by China's weapons after Chinese media boasted about Beijing's “carrier killer” missiles in response to the presence of two US Navy carrier strike groups in the South China Sea.

The Nimitz and Ronald Reagan carrier strike groups are conducting dual carrier operations in the contested waterway, over most of which Beijing claims sovereignty. The last time the US had two aircraft carriers operating in the South China Sea was nearly six years ago.

Joint operations included “air defense exercises, tactical maneuvering drills, simulated long-range maritime strike scenarios, and coordinated air and surface exercises to maintain combat readiness and maritime superiority,” the US Navy said in a statement.

On July 4, a B-52 Stratofortress bomber from Louisiana linked up with the carriers for combined operations.

The Global Times, a nationalist Chinese tabloid, argued Sunday that the “South China Sea is fully within the grasp of the Chinese People's Liberation Army,” and “any US aircraft carrier movement in the region is solely at the pleasure of the PLA.”

The incendiary paper pointed to Chinese anti-ship missiles, specifically the DF-21D and DF-26.

The US Navy responded on Twitter, writing that it is “not intimidated” by the Chinese arsenal and that the carriers are there “at our discretion.”

The Navy has repeatedly stressed that it plans to continue to operate in the South China Sea despite opposition from Beijing, a point it drove home with a meme on Instagram Sunday.

Beijing sharply criticized the dual carrier operations Monday, with the Chinese Foreign Ministry accusing the US of flexing its muscles to “undermine the peace and stability in the South China Sea.”

The two US Navy carrier strike groups were operating in the waterway at the same time Chinese naval forces were conducting military drills near the Paracel Islands, disputed territories China seized from Vietnam.

China has since constructed military outposts in the area. As China attempts to cement its sovereignty, the US Navy has repeatedly challenged China's claims, routinely sending US warships to conduct freedom-of-navigation operations in nearby waters. It did this most recently in late May.

The Department of Defense called the Chinese exercises near the Paracels “counterproductive,” characterizing the drills as “the latest in a long string of PRC actions to assert unlawful maritime claims.”

The South China Sea has long been a hotbed of tension between the US and China, both of which have stepped up their military activities in the region in recent months.

Rear Adm. George M. Wikoff, commander of the Ronald Reagan carrier strike group, told The Wall Street Journal last Friday that increased Chinese assertiveness justified the dual carrier operations.

“The purpose,” the commander said, “is to show an unambiguous signal to our partners and allies that we are committed to regional security and stability.”

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