Relatives of around 180 Rohingya refugees adrift on a boat for weeks in the Indian Ocean presume that all those on board are now dead, the UN refugee agency said.
Thousands of the mostly Muslim Rohingya, heavily persecuted in Myanmar, undertake risky sea journeys from Myanmar and refugee camps in Bangladesh every year trying to reach Malaysia or Indonesia.
This latest boat is thought to have set out last month and was reported to be drifting close to Thailand, Malaysia, Indonesia, India's Andaman Islands and the Malacca Strait, one of the world's busiest shipping routes.
"Relatives have lost contact. Those last in touch presume all are dead. We hope against hope this is not the case," the UNHCR tweeted on Sunday.
"If true, this will be devastating news. Our heart goes out to all the families who would have lost loved ones in this shocking tragedy. We repeat our pleas to the States in the region to help save lives. This must be a priority."
Last week Noor Habi, a resident of a Rohingya refugee camp in Bangladesh, said her 23-year-old daughter Munuwara Begum was on the boat and had spoken to her sister by walkie-talkie.
"We are in danger. Please save us," Begum had said, according to an audio clip of the call.
"There is no food and water with us, and there is no one to save us from this drowning boat."
On Sunday another boat -- the third in recent months -- with a broken engine and carrying 57 Rohingya refugees landed on Indonesia's western coast after a month at sea, police said.
Last week another 104 Rohingya refugees were reportedly rescued from another boat off Sri Lanka, hundreds of kilometres from Myanmar on the other side of the Bay of Bengal.
The International Organisation for Migration (IOM) this month urged states in the region "to work urgently and collectively to avoid a repeat of the 2015 crisis when thousands of men, women, and children faced tremendous challenges in accessing life-saving care and support resulting in loss of life at sea".
"Governments and partners have come together before, to address this at the regional level... With the lives and safety of refugees hanging in the balance, at the hands of smugglers, we once again call for urgent regional action," it said.