Two sources with knowledge of Sunday's meeting told Reuters that Myanmar's junta-appointed foreign minister had been invited.
Myanmar's military spokesman did not respond to phone calls on Friday night.
ASEAN chair Indonesia has declined to attend the proposed meeting, according to three sources.
Phil Robertson, deputy Asia director of Human Rights Watch, said the Thai foreign minister had shown "arrogance" by inviting his junta counterpart who other regional neighbours have shunned.
Indonesia last month cited progress in its own behind-the-scenes efforts to engage multiple parties in Myanmar's conflict in a bid to advance a peace process agreed by ASEAN leaders and Myanmar's military in April 2021.
Persons:
Don Pramudwinai, Nobel, Aung, hasn't, Myanmar's, Prayuth Chan, ocha, Don's, Don, Phil Robertson, Ananda Teresia, Devjyot Ghoshal, Kay Johnson, Stanley Widianto, Martin Petty, Devjyot, Angus MacSwan
Organizations:
ASEAN, of Southeast Asian Nations, Thailand's Foreign, Reuters, Myanmar's, Foreign Ministry, Human Rights, Party, United Nations, Thomson
Locations:
Myanmar, Thailand, Indonesia, Thailand BANGKOK, Suu, Jakarta, Asia, Bangkok, Shoon