Myanmar's government has said it is not responsible for a migrant boat crisis in south-east Asia - and it may not attend an emergency summit on it.
Thousands of migrants from Bangladesh and Myanmar are feared stranded in boats in the Andaman Sea after their crews deserted them.
Indonesia, Malaysia and Thailand have been turning away migrant boats - although, according to the ABC, the Thai and Indonesian navies restocked those they pushed back, saying the migrants did not want to come ashore.
A UN spokesperson, Rupert Colville, has described the situation as shocking.
"We're appalled at reports that Thailand, Indonesia and Malaysia have been pushing boats full of vulnerable migrants back out to sea, which will inevitably lead to many avoidable deaths. The focus should be saving lives, not further endangering them."
Survivors have described desperate conditions on the boats, with people thrown overboard amid fights for food.
Rohingya Muslims have been leaving Buddhist-majority Myanmar, also known as Burma, because they are not recognised as citizens and face persecution.
Many of the Bangladeshis at sea, meanwhile, are thought to be economic migrants.
BBC correspondent Jonah Fisher, in Bangkok, said there were at least five people-smuggling boats, carrying up to 1000 migrants, moored just off the northern coast of Myanmar near the maritime border with Bangladesh.
The crackdown on boat people landing in Thailand and Malaysia means the smugglers are now reluctant to make the journey but Mr Fisher said they were refusing to release those on board unless ransoms were paid.
Thailand is hosting a meeting on 29 May for 15 countries to discuss ways to address the crisis.
However, Zaw Htay, the director of Myanmar's presidential office, said his leaders would not attend if the word "Rohingya" was used in the invitation, as they did not recognise the term.
"We are not ignoring the migrant problem, but... we will not accept the allegations by some that Myanmar is the source of the problem," he told the Associated Press.
"The problem of the migrant graves is not a Myanmar problem, it's because of the weakness of human trafficking prevention and the rule of law in Thailand," he said in a separate interview with AFP.
Close to 800 migrants were rescued after their boat sank on Friday near Langsa in Indonesia's Aceh province, after being pushed back from both Indonesia and Malaysia's coasts.
The boat had reportedly been at sea for two months and was recently deserted by its crew.
The Rohingya and Bangladeshi migrants on board began fighting over dwindling food supplies, survivors said.
"They were killing each other, throwing people overboard," Langsa police chief Sunarya told AFP.
Rohingya Muslims attempt to flee Myanmar every year during the non-monsoon season.
The numbers involved in the current crisis are unclear but rights group have said thousands are thought to be stranded.
Who are the Rohingyas?