China's premier said on Wednesday that major powers must keep their differences under control and avoid "a new Cold War", in a thinly veiled reference to Washington, as top Asian and US officials gathered for talks in Indonesia.

Beijing has expressed concern about US-backed blocs forming on its doorstep while facing disputes with other powers in the region over the South China Sea and other issues.

"Disagreements and disputes may arise between countries due to misperceptions, diverging interests or external interferences," Li Qiang said at the start of an ASEAN-plus-three meeting with Japan and South Korea in Jakarta.

"To keep differences under control, what is essential now is to oppose picking sides, to oppose bloc confrontation and to oppose a new Cold War."

The 10-member Association of Southeast Asia Nations (ASEAN) is holding separate summits with China, Japan, South Korea, the United States and Canada on Wednesday, providing an arena for big powers to lobby the bloc and their rivalries to play out.

US Vice President Kamala Harris is attending in place of President Joe Biden, while Li was taking part instead of President Xi Jinping.

The Chinese premier's comments came after Beijing's Defence Minister Li Shangfu in June warned against establishing NATO-like alliances in the Asia-Pacific region, calling for "inclusive cooperation" instead of "small cliques" at the Shangri-La Dialogue in Singapore.

Washington is a member of the AUKUS defence partnership with Australia and Britain, and the Quad, a strategic alliance that includes Australia, India and Japan.

Wednesday's meetings come before an 18-member East Asia Summit on Thursday to be attended by Russian Foreign Minister Sergei Lavrov and the G20 summit in New Delhi at the weekend, where broader geopolitical issues are expected to top the agenda.

Harris met ASEAN leaders, praising them for their "shared commitment to international rules and norms... and regional issues".

In a sign of Washington's increasing regional engagement, she announced the creation of the first US-ASEAN centre in Washington.

Japanese Prime Minister Fumio Kishida and South Korean President Yoon Suk Yeol are attending both days of meetings.

- 'Undermine peace' -

Yoon reportedly told ASEAN leaders there must be no cooperation with North Korea, which the United States said this week is holding arms talks with Russia.

"Any attempts to forge military cooperation with North Korea... should be immediately stopped," Yoon was quoted by a presidential official as telling an ASEAN meeting, according to Yonhap news agency.

Kishida and Yoon met Li alongside ASEAN leaders, with a row between China and Japan over the release of treated wastewater from the damaged Fukushima nuclear plant one of the topics raised.

"Japan and China talked about Fukushima, but it wasn't heated," a Southeast Asian diplomat who was in the room told AFP.

Chinese state news agency Xinhua said Li outlined Beijing's position on the issue and called on Japan to handle the matter responsibly.

Japan public broadcaster NHK, citing unnamed officials, said Kishida explained Japan's stance to Li and sought Beijing's understanding.

A roundtable involving Lavrov and Harris on Thursday will be the first high-level US-Russia encounter since a foreign ministers meeting in Jakarta in July, where US and European officials rounded on Moscow's top diplomat over the Ukraine conflict.

ASEAN leaders will host summits on Thursday with Indian Prime Minister Narendra Modi, Australian Prime Minister Anthony Albanese and United Nations Secretary-General Antonio Guterres.

- Myanmar, South China Sea -

Myanmar was also a major issue at the summits with China -- a key diplomatic ally of the junta in the former Burma.

Southeast Asian leaders strongly condemned the violence and attacks on civilians in Myanmar on Tuesday, directly blaming the junta.

An ASEAN chair statement released Wednesday said the bloc was "gravely concerned" about the lack of progress on a five-point peace plan agreed with the junta two years ago.

Harris said Washington would "continue to press the regime to end the horrific violence, to release all those unjustly detained and to reestablish Myanmar's path to inclusive democracy".

China also upset several ASEAN members last week when it released a new official map claiming sovereignty over the majority of the South China Sea.

It sparked sharp rebukes from across the region, including Japan, Malaysia, Vietnam and the Philippines.

Some ASEAN leaders expressed concern about "land reclamations, activities, serious incidents" in the disputed South China Sea, according to the chair statement.

Experts, however, said ASEAN leaders would not confront Li to avoid angering Beijing.

"They won't risk the relationship with big powers," Aleksius Jemadu, a foreign affairs expert at Indonesia's Pelita Harapan University, told AFP.