South Korean President Yoon Suk Yeol says any attempts to change the status quo by force in the South China Sea cannot be tolerated, calling for a rules-based maritime order in the region.

Yoon was speaking at the East Asia Summit with the ASEAN bloc, China, the United States and others in Jakarta, Indonesia.

"(Yoon) emphasised the need for the establishment of a rules-based maritime order in the South China Sea, the region's key sea lane," his office said.

Yoon also said North Korea's nuclear programme was a real threat that can target all countries participating at the summit, and stressed the "heavy responsibility" of permanent members of the United Nations Security Council.

The remarks come amid reports North Korean leader Kim Jong Un plans to travel to Russia this month to meet President Vladimir Putin and discuss weapons supplies to Moscow.

The United States has said North Korea would pay a price for supplying Russia with weapons to use in Ukraine. South Korea has said the United Nations member states should not violate sanctions, including through arms deals.

An undercurrent of tension has accompanied the summit on issues from trade and technology to China's increasing assertiveness in the South China Sea, the Myanmar junta's refusal to co-operate with ASEAN on a peace plan, and suspicion North Korea plans to supply weapons to Russia.

On Wednesday, Chinese Premier Li Qiang warned against starting a "new Cold War" and warned countries against taking sides in any conflict.

Harris, attending the meetings instead of President Joe Biden, reiterated a US commitment to the region.

"The United States has an enduring commitment to Southeast Asia and more broadly to the Indo-Pacific," she said.

A White House official said earlier the US shared interests with ASEAN in "upholding the rules-based international order, including in the South China Sea, in the face of China's unlawful maritime claims and provocative actions".

The Chinese premier and Japanese Prime Minister Fumio Kishida met briefly on the sidelines of the summit on Wednesday and discussed Japan's release into the sea of treated radioactive water from the Fukushima nuclear plant.

An infuriated China has banned on all aquatic imports from Japan in response. It was not clear if the two would hold a bilateral meeting on Thursday.

Indonesia, chair of the 10-member ASEAN, is expected to symbolically hand over the chair to Laos on Thursday, though it will maintain the role until the end of the year.