His comments at a meeting with the 10-member Association of Southeast Asian Nations in Laos on Friday drew swift condemnation from Beijing, which blamed US and other military presences from beyond the region for instability in the waterway, which Beijing claims almost in its entirety.
China has overlapping claims with ASEAN members Vietnam, the Philippines, Malaysia and Brunei, as well as Taiwan.
A series of recent violent confrontations between China and Philippines as well as Vietnam have fuelled concerns that China's increasingly aggressive actions in the sea could spiral into a full-scale conflict.
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 opening speech at the US-ASEAN summit.
"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.
In Beijing, Chinese foreign ministry spokesperson Mao Ning said US and other military presences from outside the region were the main source of instability in the sea.
"The increasing military deployment and activities in the South China Sea by the US and a few other non-regional countries, stoking confrontation and creating tensions, are the greatest source of instability for peace and stability in the South China Sea," Mao said.
Chinese and Philippine vessels have clashed repeatedly in 2024, and Vietnam said last week that Chinese forces assaulted its fishermen in the disputed sea.
Beijing spokesperson Mao Ning blamed the US and others from outside the region for instability. (EPA PHOTO)
The US has warned repeatedly that it's obliged to defend its treaty ally the Philippines 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'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.
Officials have agreed to try to complete the code by 2026, but talks have been slow.
In another firm message to China, Blinken said the US is also committed to help protect stability across the Taiwan Strait.
The US has stepped up support for Taiwan, including new military financing in September that sparked protest from China, which claims the self-ruled island as its own territory.
The 18-nation East Asia Summit took place during the ASEAN meeting in Vientiane, Laos. (AP PHOTO)
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.
ASEAN has treaded carefully on the sea dispute with China, which is the bloc's largest trading partner.
It has not marred trade relations, with the two sides focusing on expanding a free-trade area covering a market of two billion people.
Blinken said the 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.
The Middle East crisis was also raised, with pro-Palestine Muslim countries such as Malaysia and Indonesia repeating calls for an immediate Israeli ceasefire as the war marked its one-year anniversary on Monday.
Blinken also said Israel's escalated campaign in Lebanon against Hezbollah has "clear and legitimate" reasons, but said the US was trying to find a diplomatic solution to the war.
