A United Nations special envoy has urged Southeast Asian countries to support international efforts to engage all sides in the crisis in army-ruled Myanmar, days after a top regional leader travelled there to meet its junta chief.

Noeleen Heyzer, the secretary-general's special envoy on Myanmar, held virtual talks with Cambodian Prime Minister Hun Sen, the new chairman of the Association of Southeast Asian Nations (ASEAN) and sought a collaborative effort in humanitarian aid and seeking progress in a stalled five-point peace plan, the UN said in a statement on Thursday.

Hun Sen visited junta boss Min Aung Hlaing last week, a move rights groups said risked legitimising the military's coup last year and its crackdown on thousands of democracy activists and supporters of Aung San Suu Kyi's ousted government.

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".

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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 organisations," the statement said of Heyzer's discussion with Hun Sen.

The conflict has caused discord within ASEAN about how to deal with last year of its top general from ASEAN meetings over a failure to honour peace commitments.

An envoy from the previous chair, Brunei, made meeting all stakeholders a precondition for visiting, which the junta rejected. Cambodia's incoming Myanmar envoy Prak Sokhonn said that approach was not productive.

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

"She emphasised solutions needed to derive from engaging directly with and listening carefully to all those affected," it said.