USS Benfold entered the waters of the Paracels on Monday without the approval of the Chinese government, seriously violating China's sovereignty and undermining the stability of the South China Sea, the People's Liberation Army's Southern Theatre Command said. 

"We urge the United States to immediately stop such provocative actions," the Southern Theatre Command said in a statement.

The US Navy did not immediately comment.

The incident came as the US repeated a warning to China that an attack on Philippine armed forces in the South China Sea would trigger a 1951 defence treaty between the US and the Philippines.

US Secretary of State Antony Blinken made the comment in a written statement exactly five years after the ruling by an arbitration tribunal repudiating China's vast territorial claims in the South China Sea.

Activists at Makati City, near Manila, staged a rally outside the Chinese consulate, guarded by Philippine police, to show their displeasure over Beijing's ongoing aggression in the region.

The Paracels are among hundreds of islands, reefs and atolls in the resource-rich South China Sea.

They are contested by China, Vietnam, Taiwan, Philippines, Malaysia and Brunei, with Beijing claiming historic rights to resources within its so-called nine-dash line, or most of the region.

On July 12, 2016, the Permanent Court of Arbitration in The Hague ruled that China had no historic title over the South China Sea.

It also said China had interfered with traditional Philippine fishing rights at Scarborough Shoal and breached the nation's sovereign rights by exploring for oil and gas near the Reed Bank.

China has rejected the tribunal's decision, dismissing it as a “sham”, and has refused to participate in arbitration proceedings. 

In a written statement on Sunday, Mr Blinken said freedom of the seas was an "enduring" interest of all nations.

 "Nowhere is the rules-based maritime order under greater threat than in the South China Sea," Mr Blinken said.  

"The People's Republic of China continues to coerce and intimidate South-East Asian coastal states, threatening freedom of navigation in this critical global throughway."

Although the US continues to remain neutral in territorial disputes, it has effectively sided with China's South-East Asian neighbours.

Sources | ABC / Wires