If another 200,000 are pushed out it would virtually fulfil Myanmar's long-term goal to depopulate the Rakhine State of the Rohingya minority group.
Turkey has been at the forefront of providing aid to Rohingya refugees.
Pramila Patten, the United Nations special representative of the secretary-general on sexual violence in conflict, said she would raise accusations against the Burmese military with the International Criminal Court in the Hague.
But in its report on Monday, the Tatmadaw claimed that there had been "no deaths of innocent people".
An internal army investigation claims Myanmar soldiers have not shot villagers, raped women or tortured prisoners.
The refugees are fleeing a military operation in which security forces and Buddhist mobs have killed men, women and children, looted homes and torched Rohingya villages.
The report stated that almost 376 "terrorists" were killed in fighting during the August attacks.
Guterres, addressing the summit, described the exodus of refugees from Burma as a "worrying escalation in a protracted tragedy" and a potential source of instability and radicalisation in the region.
Canadian Prime Minister Justin Trudeau on Tuesday added his voice to the chorus of concern, informing ASEAN leaders he has tasked his special envoy to engage in diplomatic efforts to resolve the Rohingya crisis.
"Exploitation has been reported among Rohingya refugees not only among those who arrived since August 25 this year but also among those who have lived in Bangladesh for years".
Parmaukkha is one of the most prominent nationalist monks in the country.
With fewer boats available, desperate Rohingya have been stringing together rafts from bamboo and plastic canisters. He is expected to meet senior officials to hold talks on the crisis.
President Donald Trump failed to speak out about two of Asia's biggest humanitarian crises ― the ethnocentric violence in Myanmar and the alleged extrajudicial killings in the Phillippines ― during his 12-day visit to the area, even as some of America's closest allies delivered strongly worded responses.
"That's always our ultimate approach, not to judge but to offer help and to see how we can make the whole region a region of peace and stability, a region where human rights is respected", Cayetano said.
"It is something for which the Burmese authorities ― and especially the military ― must take full responsibility", May said.
"There is overwhelming evidence that the military has murdered and raped Rohingya and burned their villages to the ground", Amnesty said in a statement released on Monday evening.
"The Burmese military's absurd effort to absolve itself of mass atrocities underscores why an independent worldwide investigation is needed to establish the facts and identify those responsible", said Brad Adams, Asia director at HRW.