The US-China rivalry and growing fears of a new North Korean nuclear test will loom over a meeting this week of Southeast Asian leaders attended by US President Joe Biden.
Leaders from the Association of Southeast Asian Nations (Asean) will meet in Phnom Penh from Friday, kicking off a diplomatic blitz in the region that also takes in next week's G20 in Bali and an Apec summit in Bangkok.
Biden's administration has identified China as the only global rival to the United States, saying Beijing is attempting to remould the world order in "its own authoritarian model".
Making his second trip to Asia this year off the back of bruising midterm elections at home, Biden faces another tough battle to woo Asean leaders, many of whom are wary of overtly taking sides against a giant neighbour and key trading partner.
A senior US official said Biden would push the importance of peace in the region, including Taiwan, and respect for the "rules-based international order".
An Asean foreign ministers' meeting in August was dominated by US House Speaker Nancy Pelosi's visit to Taiwan and the furious response from Beijing, which claims the self-governing island as part of its territory.
Chinese Premier Li Keqiang stole a diplomatic march on Biden on Tuesday, arriving in Phnom Penh for talks with Cambodian leader Hun Sen.
At a summit with Asean leaders in Washington in May, Biden pledged $150 million in support for Southeast Asian nations -- dwarfed by the $1.5 billion that China promised to the region last year in Covid aid alone.
The US-China relationship is deteriorating over issues including Taiwan and alleged rights abuses in Xinjiang, and other Western governments have complained about Chinese activities on their soil, including harassment of dissidents.
Chinese President Xi Jinping has said Beijing and Washington must "find ways to get along", but at the same time has continued to enforce a far more muscular foreign policy that shows no deference to the United States.
Xi is expected to attend the G20 summit in Bali, where he will have his first face-to-face meeting with Biden on its sidelines.
In Phnom Penh, Li can expect a warm reception, having cultivated close ties with most Asean members, including host Cambodia.
"China will seek to consolidate her relationships with Southeast Asian countries, in order to either shore up regional support for Beijing or make sure that they do not end up being on the US side against China," analyst Yongwook Ryu of Singapore's Lee Kuan Yew School of Public Policy told AFP.
The war in Ukraine is also likely to weigh on leaders' minds at Asean.
Kyiv will sign a "treaty of amity and cooperation" with Asean on Thursday, a first step towards establishing formal relations with the bloc.
And Cambodia has said it is considering a request by President Volodymyr Zelensky to address the meeting by video link.
China has refused to join Western sanctions on Russia, and Washington has accused Beijing of providing diplomatic cover for Moscow's invasion of Ukraine.
Russia has been invited to the Asean summit but it is not clear who will represent it.
- N. Korea missile drama -
Leaders are also expected to discuss the growing crisis on the Korean peninsula, where Pyongyang carried out a spate of weapons tests last week -- including an intercontinental ballistic missile.
Washington and Seoul have warned that the launches could culminate in a nuclear test, which would be the reclusive communist state's seventh.
North Korea, whose closest ally is China, has said its tests were in response to the largest-ever military drills between the United States and South Korea, which ended at the weekend.
Biden is also expected to hold talks with counterparts South Korean Yoon Suk-yeol and Japanese Prime Minister Fumio Kishida, either on the sidelines of Asean or at the G20, according to Japanese media.
Asean leaders will also tackle Myanmar, where bloody conflict rages between the junta, which seized power in February last year, and civilian militias trying to oust it.
Leaders of the 10-member bloc, which has led so far fruitless diplomatic efforts to resolve the crisis, are expected to discuss ways to implement a "five-point consensus" peace plan agreed upon with Myanmar last year.
