Blinken warned on Tuesday that Washington was obligated to defend treaty ally Philippines if its forces, vessels or aircraft come under attack in the disputed waters.
His statement, issued by the US Embassy in Manila was released on the sixth anniversary of the 2016 decision by an arbitration tribunal set up in The Hague under the United Nations Convention on the Law of the Sea after the Philippine government complained in 2013 against China's increasingly aggressive actions in the disputed sea.
China did not participate in the arbitration, rejected its ruling as a sham and continues to defy it, bringing it into territorial spats with the Philippines and other Southeast Asian claimant states in recent years.
"We call again on the PRC to abide by its obligations under international law and cease its provocative behaviour," Blinken said, using the acronym of China's formal name.
"We also reaffirm that an armed attack on Philippine armed forces, public vessels, or aircraft in the South China Sea would invoke US mutual defence commitments" under the 1951 US-Philippines Mutual Defence Treaty, Blinken said.
Aside from China and the Philippines, Vietnam, Malaysia, Taiwan and Brunei have had overlapping claims in the busy waterway, which is believed to be rich in undersea gas and oil deposits and where an estimated $US5 trillion-worth of trade and goods are shipped through each year.
The flashpoint region has become a key front of US-China rivalry.
Washington lays no claim to the disputed waters but has deployed its Navy ships and Air Force jets to patrol the waterway for decades and says freedom of navigation and overflight in the disputed region is in the US national interest.
That has provoked angry reactions from China, which has accused the US of meddling in a purely Asian dispute and warned it to stay away.
Philippine Foreign Secretary Enrique Manalo said on Tuesday that the arbitration ruling would be a pillar of the new administration's policy and actions in the disputed region and rejected attempts to undermine the "indisputable" decision.
"These findings are no longer within the reach of denial and rebuttal and are conclusive as they are indisputable. The award is final," Manalo said in a statement.
China would likely frown on the policy stance declared by Manalo for the administration of President Ferdinand Marcos Junior, who took office June 30 after a landslide electoral victory.
Marcos Junior's predecessor, Rodrigo Duterte, put the arbitration ruling in the backburner for years after he took office in 2016 and nurtured cozy ties with Chinese President Xi Jinping while often criticising US security policies.
In 2019, Duterte said he finally asked Xi in a meeting in Beijing to comply with the ruling but was flatly told by the Chinese leader: "We will not budge."
Marcos Junior has upheld the arbitration ruling and said he would not allow even one "square millimetre" of Philippine waters to be trampled upon.
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