Also read: ASEAN Foreign Ministers Hold Emergency Talks on Myanmar
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.
North 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.
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.
Source: Agence France-Presse
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