China’s military said on Tuesday it drove away a U.S. guided-missile cruiser that “illegally intruded” into waters near the South China Sea’s Spratly Islands.

“The actions of the U.S. military seriously violated China’s sovereignty and security,” said Tian Junli, spokesman for the Southern Theatre Command of the People’s Liberation Army.

The ship in question, the USS Chancellorsville guided missile cruiser, had recently sailed through the Taiwain Strait.

There was no immediate comment from the U.S. military.

Tian said the U.S. cruiser’s intrusion showed that the United States was a “security risk maker” in the South China Sea and “is another iron-clad proof of its hegemony in the navigation and militarization of the South China Sea”.

China’s military said its troops would remain on high alert, the Southern Theatre Command said on its WeChat social media account.

China claims nearly all of the South China Sea and it has become one of many flashpoints in the testy relationship between it and the United States.

The United States rejects what it calls China’s unlawful territorial claims in the resource-rich waters.

U.S. warships have passed through the South China Sea with increasing frequency in recent years, in a show of force against the Chinese claims.