Only Cambodia has so far officially confirmed it intended to attend the talks.
Myanmar's junta spokesman could not be reached for comment on Sunday.
Thailand's foreign ministry was tight-lipped about exactly who was attending the two-day gathering in the resort town of Pattaya, for which outgoing Foreign Minister Don Pramudwinai sent invitation letters just four days before its start.
Cambodian Foreign Minister Prak Sokhonn was to attend the meeting, his government said in a statement on Friday.
Vietnam's government said its foreign minister would not attend "due to a prior engagement".
Persons:
Nobel, Aung, Myanmar's, Swe, Don Pramudwinai, Don, Prak Sokhonn, Vivian Balakrishnan, Nantiwat Samart, Suu Kyi's, Panu, Phuong Nguyen, Ananda Teresia, Stefanno Sulaiman, Poppy McPherson, Devjyot, Kay Johnson, William Mallard
Organizations:
Association of Southeast Asian Nations, ASEAN, Reuters, Cambodian, Nation TV, National Unity Government, Thai, Ananda, Thomson
Locations:
BANGKOK, Cambodia, Suu, Thailand, Pattaya, Myanmar, Indonesia, Singapore, Thai, ASEAN, Malaysia, Philippines, Bangkok, Hanoi, Kuala Lumpur, Jakarta, Shoon