Cambodia and the other ASEAN member states “are deeply disappointed and disturbed by the execution of those opposition activists, despite the appeals from me and others for the death sentences to be reconsidered”, said Hun Sen.
Myanmar's military last week defended the execution of the activists as “justice for the people”, brushing off a deluge of international condemnation, including by its closest neighbours.
The military said it had executed the activists for aiding “terror acts” by a civilian resistance movement, Myanmar's first executions in decades.
Myanmar will not be represented at this week's meeting, a spokesperson for the ASEAN chair said on Monday, after its military rulers declined a proposal to send a non-junta representative instead.
ASEAN has since late last year barred the Myanmar junta from joining its meetings due to its lack of progress in implementing the peace plan.
Some other members of ASEAN, which has a tradition of non-interference in each other's internal affairs, have been increasingly strident in their criticism of the generals.
Malaysian foreign minister Saifuddin Abdullah has described the executions as a crime against humanity that appear to make “a mockery” of the ASEAN peace plan.
The head of Myanmar's junta, Min Aung Hlaing, on Monday blamed instability related to the pandemic and internal violence for stalling efforts to implement the peace plan.
The junta also extended a state of emergency put in place after seizing power from the elected government of Aung San Suu Kyi in February last year.
Myanmar has been in chaos since then, with conflict spreading after the army crushed mostly peaceful protests in towns and cities.