MANILA (REUTERS) - Washington will defend treaty ally the Philippines if its forces are attacked in the South China Sea, US Secretary of State Antony Blinken has said.

Mr Blinken urged China to follow international law and cease "provocative behaviour" in the busy waterway.

He made the comments on Monday (July 11), the sixth anniversary of a ruling by an international tribunal that invalidated China's sweeping claims to the South China Sea, a conduit for about US$3 trillion (S$4.2 trillion) worth of ship-borne trade each year.

"We reaffirm that an armed attack on Philippine armed forces… would invoke US mutual defence commitments," Mr Blinken said in a statement, referring to the provisions of a mutual defence treaty between the allies dating back to 1951.

"We call again on the People's Republic of China to abide by its obligations under international law and cease its provocative behaviour," he added.

The statement came the day that Chinese Foreign Minister Wang Yi told a meeting of the Association of Southeast Asian Nations in Indonesia that regional nations should avoid being used as "chess pieces" by global powers.

China's claim to almost the entire South China Sea, citing what it says are historical maps, puts it at odds with some countries in the grouping, which say the claim is inconsistent with international law.