* If no progress, limited value in engaging junta - Singapore
* Malaysia says junta frustrating ASEAN
* ASEAN calls for de-escalation of tensions over Taiwan
* U.S. seeking to sustain, strengthen pressure on Myanmar (Adds comments from Malaysia foreign minister, Singapore, and ASEAN chair and Myanmar Taiwan tensions)
PHNOM PENH, Aug 3 (Reuters) - The Association of Southeast Asian Nations (ASEAN) will be forced to reconsider a peace plan agreed with Myanmar if its military rulers execute more prisoners, Cambodian Prime Minister Hun Sen said on Wednesday.
The 10-nation bloc, which is hosting a big international gathering this week, has pushed the Myanmar junta to follow a peace "consensus" agreed last year and has condemned its recent execution of four activists linked to a resistance movement, its first executions in decades.
"If more prisoners are executed, we will be forced to rethink...our role vis a vis ASEAN's five-point consensus," Hun Sen, the chair of ASEAN, said in opening the meeting of its foreign ministers.
He said ASEAN's unity had been challenged by the Myanmar crisis, and while the peace plan had "not advanced to everyone's wishes" there had been some progress on providing humanitarian aid.
But Hun Sen said the situation had changed dramatically and was even worse than before the peace agreement, due to the junta's execution of the activists, which he said ASEAN states "are deeply disappointed and disturbed" by.
A senior State Department official on Wednesday said the United States was "looking what can be done to both sustain and increase the pressure" on the Myanmar generals.
U.S. Secretary of State Antony Blinken is among 27 foreign ministers set to join the ASEAN Regional Forum security meeting in Cambodia on Friday, which will also include counterparts from China, Russia, Japan, Britain and Australia.
The Myanmar junta last week defended the executions as "justice for the people", brushing off a deluge of international condemnation.
Those activists had aided "terror acts", it said, referring to attacks by militias fighting military rule and seeking to avenge a deadly crackdown on protests.
GENERALS BARRED
Myanmar is not represented at this week's ASEAN meetings after its military declined a proposal to send a non-junta representative. ASEAN has barred the generals until progress in the peace plan is demonstrated.
Some members of ASEAN, which has a tradition of non-interference in each other's internal affairs, have been increasingly strident in their criticism of Myanmar.