US ship illegally entered territorial waters: China

"The US seriously undermined regional peace and stability," a spokesperson for China's Southern Theater of Operations said in a statement on Monday.

The spokesperson said the US disrupted the South China Sea and violated China's sovereignty.

China is in dispute with several of its neighbours over its extensive claims of territorial waters in the South China Sea.

The US Navy said the USS Gabrielle Giffords, an Independence-class littoral combat ship, was conducting routine operations in international waters in the South China Sea, consistent with international law.

"Every day the US 7th Fleet operates in the South China Sea, as they have for decades," the US Navy said in a statement on Monday.

"These operations demonstrate we are committed to upholding a free and open Indo-Pacific region."

In recent months, China has had several run-ins with Philippine vessels and protested about US ships patrolling disputed areas.

According to the Chinese military, the US ship moved into waters adjacent to what China calls the Renai Reef, also known as Second Thomas Shoal, a part of the Spratly Islands.

The Second Thomas Shoal lies in the Philippines' exclusive economic zone, according to a United Nations tribunal ruling in 2016.

The Chinese military spokesperson said the US ship was monitored and followed, and that China's "troops in the theatre are on high alert at all times to resolutely defend national sovereignty".

"We will not be deterred from continuing to work alongside our allies and partners in support of our shared vision for a free and open Indo-Pacific," the US Navy said.

On Sunday, the Philippine Coast Guard deployed two of its vessels in the South China Sea after monitoring an "alarming" increase in the number of Chinese maritime militia vessels at a reef within the country's exclusive economic zone.

Australian Associated Press