Myanmar migrants on a boat stranded for a week in the Andaman Sea with no food or water say 10 people have died, while some are resorting to drinking urine.
The fishing boat, carrying about 350 people of the Muslim Rohingya minority, has been refused entry to Thailand.
Those on board told the BBC the crew abandoned them and disabled the engine. They said the bodies of those who had died were thrown overboard.
Indonesia, Malaysia and Thailand have been turning away migrant boats.
Several thousand people are still believed to be stuck in boats off the coasts of Thailand and Malaysia.
Most are Rohingya Muslims who cannot go back to Myanmar, also known as Burma, where they are not recognised as citizens of the country and are regularly persecuted.
The BBC's Jonathan Head reports from alongside the vessel off the southern coast of Thailand, off Koh Lipe, that it is a "desperate sight".
He said: "People are calling out to us begging us for food and water.
"There are a lot of women and children on board. This is a very old-looking fishing boat that's completely packed with people.
"We can see there are actually people drinking their own urine from bottles. We've been throwing them bottles of water - everything we've got on board."
He said blankets had been tied up to try and provide some shelter from the sun. The average maximum temperature is 34C.
The migrants - including 50 women and 84 children - said they had been at sea for three months.
Their situation became critical when their crew abandoned them without a working engine six days ago anchored near the Thai-Malaysian border.
On Wednesday night Thai fishing boats found the boat and it was towed into Malaysian waters.
It was then towed back to Thai waters, our correspondent reports.
A Thai Navy colonel told him the migrants had intended to reach Malaysia, and Thailand would give them food, water and medical attention and let them go on their way.
Our correspondent said that could mean they are rejected again and they are effectively in a "tug of war" between the countries.
Who are the Rohingyas?
- Rohingyas are a distinct, Muslim ethnic group mainly living in Myanmar, which is also known as Burma
- Thought to be descended from Muslim traders who settled there more than 1,000 years ago
- Also live in Bangladesh, Saudi Arabia and Pakistan
- In Myanmar, they are regularly persecuted - subjected to forced labour, have no land rights, and are heavily restricted
- In Bangladesh many are also desperately poor, with no documents or job prospects
Thailand has launched a crackdown to disrupt people smuggler networks since the discovery of dozens of bodies in abandoned camps along regular trafficking routes.
As many as 8,000 migrants from Bangladesh and Myanmar are believed by the International Organisation for Migration (IOM) to be stranded at sea.
People smugglers are reportedly refusing to land their boats because they do not want to follow their usual route through Thailand since the government's campaign against them began.
A senior Thai official told Reuters news agency on Wednesday that Thailand, Malaysia and Indonesia would all continue to turn the boats away.
Major General Werachon Sukhondhapatipak said that the three countries had decided "not to receive boat people".
On Sunday and Monday more than 2,000 migrants arrived in Malaysia or Indonesia after being rescued or swimming ashore.
The journey the migrants take - from Bangladesh or Myanmar through the Bay of Bengal to Thailand or beyond - takes several weeks. They have been slowed further by the refugees effectively being held hostage in many cases by smugglers.