The 10-member Association of Southeast Asian Nations' meeting with Blinken followed a series of violent confrontations at sea between China and ASEAN members Philippines and Vietnam, which have fuelled concerns that China's increasingly assertive actions in the waterways could spiral into a full-scale conflict.
China, which claims almost the entire sea, has overlapping claims with ASEAN members Vietnam, the Philippines, Malaysia and Brunei, as well as Taiwan.
About a third of global trade transits through the sea, which is also rich in fishing stocks, gas and oil.
Beijing has refused to recognise a 2016 international arbitration ruling by a UN-affiliated court in The Hague that invalidated its expansive claims, and has built up and militarised islands it controls.
Chinese and Philippine vessels have clashed repeatedly in the South China Sea. (AP PHOTO)
"We are very concerned about China's increasingly dangerous and unlawful activities in the South China Sea which have injured people, harm vessels from ASEAN nations and contradict commitments to peaceful resolutions of disputes," said Blinken, who is filling in for President Joe Biden, in his speech at the US-ASEAN summit in Laos.
"The United States will continue to support freedom of navigation, and freedom of overflight in the Indo-Pacific."
The US has no claims in the South China Sea, but has deployed navy ships and fighter jets to patrol the waters in a challenge to China's claims.
Chinese and Philippine vessels have clashed repeatedly in 2024, and Vietnam said last week that Chinese forces assaulted its fishermen in the disputed sea.
The US has warned repeatedly that it is obliged to defend the Philippines - its oldest treaty ally in Asia - if Filipino forces, ships or aircraft come under armed attack.
Philippine President Ferdinand Marcos Jr complained to summit leaders on Thursday that his country "continues to be subject to harassment and intimidation" by China.
He said it was "regrettable that the overall situation in the South China Sea remains tense and unchanged" due to China's actions, which he said violated international law.
He has called for more urgency in ASEAN-China negotiations on a code of conduct to govern the South China Sea.
Premier Li Qiang repeated China's claim it was merely protecting its rights in the disputed sea. (AP PHOTO)
Officials have agreed to try to complete the code by 2026, but talks have been hampered by sticky issues including disagreements over whether the pact should be binding.
Chinese Premier Li Qiang was defiant during talks on Thursday, calling the South China Sea a "shared home" but repeating China's assertion that it was merely protecting its sovereign rights.
Li also blamed meddling by "external forces" who sought to "introduce bloc confrontation and geopolitical conflicts into Asia".
Li did not name the foreign forces, but China has previously warned the US not to meddle in the region's territorial disputes.
In another firm message to China, Blinken said the US believed "it is also important to maintain our shared commitment to protect stability across the Taiwan Strait".
China claims the self-ruled island of Taiwan as its own territory and bristles at other countries' patrolling the strait separating it from the island.
Blinken also attended an 18-nation East Asia Summit, along with the Chinese premier, Russian Foreign Minister Sergei Lavrov and leaders from Japan, South Korea, India, Australia and New Zealand.
Blinken said the annual ASEAN summit talks were a platform to address other shared challenges including the civil war in Myanmar, North Korea's "destabilising behaviour" and Russia's war aggression in Ukraine.
