The meeting was to be the first under Cambodia's chairmanship of the 10-member bloc, which comes amid divisions on how to deal with the fallout of last year’s military coup in Myanmar and the ensuing bloody crackdown on thousands of the junta’s opponents.

Cambodian Prime Minister Hun Sen met Myanmar's military ruler Min Aung Hlaing last week, the first such visit by a head of government, sparking concern that it could undermine international efforts to isolate the junta.

"The postponement is because many ASEAN ministers are having difficulties to travel to join," Cambodia foreign ministry spokesperson Koy Kuong told reporters, without elaborating.

Asked separately by Reuters which ministers could not attend the Jan 18 to 19 meeting in Siem Reap and why, Koy Kuong said he could not “speak for them".

Under Brunei's chairmanship, ASEAN late last year took the unprecedented step of sidelining Min Aung Hlaing from its annual leaders' summit over his failure to honor commitments he made toward ending violence and starting a dialogue process.

The exclusion angered the junta, which said outside powers had pressured ASEAN to break its own code of consensus and noninterference.

Brunei, Singapore, the Philippines, Malaysia and Indonesia had backed excluding the junta.

Cambodia, however, is taking "different approaches", its foreign minister, Prak Sokhonn, said on Saturday, while denying that Hun Sen's visit was an endorsement of the Myanmar military.

Prak Sokhonn was expecting to be appointed special ASEAN envoy for the Myanmar situation at the Siem Reap meeting.

On Saturday, he criticized the previous envoy, Erywan Yusof, as being unproductive in insisting on access to ousted leader Aung San Suu Kyi, who has been convicted by the junta in recent weeks of several offenses, including incitement.

Meanwhile, a United Nations special envoy has urged Southeast Asian countries to support international efforts to engage all sides of the crisis in Myanmar.

Noeleen Heyzer, the secretary-general's special envoy on Myanmar, held virtual talks with Hun Sen and sought a collaborative effort in providing humanitarian aid and seeking progress in a stalled five-point peace plan, the UN said in a statement on Thursday.

Rights groups say Hun Sen’s visit to Min Aung Hlaing risks legitimizing the military coup and its crackdown on thousands of democracy activists.

Myanmar has been in chaos for nearly a year, with the military suppressing protests and fighting on different fronts with ethnic minority armies and newly formed militias it calls "terrorists".

At least 1,400 civilians have been killed, according to activists cited by the UN.

"The special envoy advocated for confidence-building measures involving all stakeholders, in addition to ethnic armed organizations," the UN statement said of Heyzer's discussion with Hun Sen.

Heyzer urged Prak Sakhonn work with her and the international community on "a coordinated strategy toward creating an enabling environment for inclusive dialogue."

"She emphasized solutions needed to derive from engaging directly with and listening carefully to all those affected," the statement said.