China’s military announced on Monday that a US Navy ship had illegally entered waters adjacent to Second Thomas Shoal, a disputed atoll in the South China Sea that has recently been the subject of several naval clashes, Reuters reported.

“The United States has seriously undermined regional peace and stability,” a spokesman for China’s Southern Theater of Military Operations said, adding that the United States had violated the South China Sea and violated China’s sovereignty, News.ro reported.

China is at loggerheads with several of its neighbors over its claims to territorial waters in the South China Sea.

The US Navy said the USS Gabrielle Giffords, an Independence-class battleship, was conducting routine operations in international waters of the South China Sea in accordance with international law.

“Every day, the US 7th Fleet operates in the South China Sea, as it has for decades,” the US Navy said in a statement. “These operations demonstrate our commitment to maintaining a free and open Indo-Pacific region.”

China has had several clashes with Philippine ships in recent months and has protested US ships patrolling disputed areas.

According to the Chinese military, the US ship entered waters adjacent to what China calls Renai Reef, also known as Second Thomas Shoal, which is part of the Spratly Islands. According to a 2016 UN tribunal ruling, the Second Thomas Shoal is in the Philippines’ exclusive economic zone.

A Chinese military spokesman said the US ship was being monitored and tracked, and that “Chinese forces in the theater of operations are constantly on high alert to resolutely defend national sovereignty.”

The Philippine Coast Guard on Sunday deployed two of its ships in the South China Sea after observing an “alarming” increase in the number of Chinese marine police ships on a reef in the country’s exclusive economic zone.