Myanmar's navy has intercepted a boat packed with 727 people off the country's southern coast, about a week after it found a similar vessel carrying around 200 migrants.
Most of those on board this boat were from Bangladesh, a senior official from the president's office, Zaw Htay, told Reuters news agency on Friday.
Myanmar's Ministry of Information described them as "Bengalis", using the term the government uses to describe the country's persecuted Muslim Rohingya minority as well as people from Bangladesh.
Earlier in the day, at a regional crisis meeting in Thailand aimed at tackling the Southeast Asian migrant crisis, Myanmar insisted it was not to blame for Southeast Asia's latest influx of "boat people".
The UN says thousands of vulnerable migrants remained adrift at sea and need urgent rescue.
More than 4,000 migrants have landed in Indonesia, Malaysia, Thailand, Myanmar and Bangladesh since Thailand launched a crackdown on human-trafficking gangs this month.
About 2,000 are believed to be still adrift.
Pictures posted on the Myanmar Ministry of Information's Facebook page showed scores of men huddling shoulder to shoulder under the sun on the front deck, while uniformed officials - one of them carrying a rifle - stood above.
Women could be seen crammed together in the boat's cabins.
The navy found the boat 30 nautical miles off Myanmar's southern coast in the Andaman Sea.
The government said there were 608 men, 74 women and 45 children on board.
The ministry later said that authorities found after questioning those on board that they had been at sea since March and at least 50 people had died in that time.
They had set off on three boats and had waited in Thai territorial waters since April to be picked up by traffickers who would take them to Malaysia, but when no one turned up, they came back.
The 11 crew members, identifed as Thai nationals, then crammed the migrants on one boat and left on the other two vessels, the ministry said in a statement on its website.
"Arrangements will be made to repatriate the people found on the boat to their country after necessary investigation," it said.
At Friday's meeting in Bangkok, the UN refugee agency and Myanmar quarrelled over the plight of the Rohingya ethnic minority. Myanmar's delegate to the talks rebuked the UNHCR for calling on the country to recognise the Muslim Rohingya minority as citizens to stem their exodus from its shores.
The majority of migrants are Rohingya Muslims fleeing persecution in Buddhist-majority Myanmar or Bangladeshis trying to escape poverty.
On "this issue of illegal migration of boat people, you cannot single out my country", Htin Lynn, foreign ministry director-general, said.
Earlier, Volker Turk, the UN's assistant high commissioner for refugees, said there could be no solution if root causes of the migrant crisis were not addressed.
Myanmar's government does not recognise the Rohingya - who make up the majority of the migrants involved in the current crisis - as an ethnic group, arguing instead they are really Bangladeshis.
Bangladesh also does not recognise the Rohingya as citizens.
According to Human Rights Watch, Rohingya - who have lived in Myanmar for generations - are victims of an ongoing ethnic cleansing.
The word "Rohingya" did not appear on the invitation to Friday's meeting, after Myanmar threatened to boycott the talks if it did. The Rohingya people were excluded from the latest Myanmar census.
Al Jazeera's Scott Heidler, reporting from Bangkok, said the meeting was seen as a step forward to the extent that the Myanmar government was represented.
"[Representatives at the meeting] are very much focused on helping the people still stranded out at sea," he said, adding that they were looking to coordinate search efforts.
He said the US and Australia donated $6m to help deal with the crisis.
Thailand said it had given the US permission to fly surveillance flights over Thai airspace to identify boats carrying migrants as part of the rescue efforts. While Malaysia and Indonesia have agreed to temporarily house migrants that have been rescued, Thailand has so far said it will only assist by offering medical aid at sea.
Source: Al Jazeera and agencies