BANGKOK: Thousands of migrants and asylum seekers are stranded in boats at sea with no place to go as Indonesia and Malaysia refuse to offer them refuge in an escalating humanitarian crisis in South-east Asia.

Now a 15-year-old Rohingya asylum seeker has told how he is stranded with dead people on a fishing trawler that Malaysia and Thailand are refusing to allow to come ashore.

"There are 400 on board. Some are already dead but I cannot tell you how many because I do not have the strength to move around the ship," Abdul Rahamad told a journalist from the Phuketwan online news website by mobile phone.

"We have been at sea for a month," he said in a weak voice in the Rohingya language.

"This is a Thai trawler modified to carrying people," he said. "We are not sure where we are. We have used up most food and water - we beg for your help."

The teenager said the last he heard 22 people had died on board the ship.

"It's very crowded - many people are exhausted and unable to move," he said, speaking on one of three mobile phones provided by people smugglers allowed to arrange ransom money from their relatives.

He said the two smugglers left the ship in a small boat that came to pick them up.

"I can see mountains on an island. It looks about an hour away," he said. "We must still be in reach of land."

The United Nations refugee agency has urged South-East Asian nations to initiate a search and rescue operation for thousands of Rohingya and Bangladesh migrants and asylum seekers stranded at sea on boats that have crossed the Bay of Bengal from Myanmar and Bangladesh in the largest movement of people in the region since the Vietnam War.

Since more than 1500 migrants came ashore in Indonesia and Malaysia on Sunday and Monday both countries said they would push back out to sea any more boats that arrive without permission.

Thailand has arranged a summit of regional countries on May 29 to discuss what refugee advocates describe as an escalating humanitarian crisis.

Malaysia has bluntly told long-persecuted Rohingya Muslims fleeing Myanmar they are not welcome after for years allowing tens of thousands of them to quietly come ashore and live among Malaysians.

"We won't let any foreign boats come in," said Tan Kok Kwee, first admiral of Malaysia's maritime enforcement agency.

Unless the boats were unseaworthy and sinking, he said, the navy would provide "provisions and send them away".

The declaration came hours after Indonesia, appearing to have adopted Canberra's boats-tow-back policy, pushed back a ship carrying hundreds of Rohingya and Bangladeshi migrants and asylum seekers, including women and children, after providing fuel, food and water.

"They should not have entered Indonesian waters without our permission," said Fuad Basya, a spokesman for the Indonesian Army.

Amid growing concern for people abandoned at sea by people smugglers, many of whom are believed to have little food, water or medicines, the United Nations refugee agency called on regional governments to keep their borders open and initiate search and rescue operations to find the boats.

Adrian Edwards, spokesman for the United Nations High Commissioner for Refugees (UNHCR), appealed to governments "to continue their lifesaving operations to find and safely disembark the passengers, many of whom are believed to be in a weakened state after days, possibly weeks, with little food and water"."

The multi-nation International Organisation for Migration (IOM) said search and rescue operations were urgently needed.

"It needs a regional effort ... we don't have the capacity to search for them, but governments do," IOM spokesman Joe Lowry said.

"They have boats and satellites," he said.

Those at sea may be in "very bad condition if not dead" if not found soon, Mr Lowry said.

Chris Lewa, of the Arakan Project, who for years has tracked boats making the dangerous crossing of the Bay of Bengal, estimates that between 6000 and 20,000 people are still in peril at sea.

She has been in touch by mobile telephone with passengers on one boat that ran out of food three days ago.

They can see land but have no idea where they are.

"You just can't let these people die at sea," she said.

Tens of thousands of Rohingya have fled Myanmar over the past three years where they have suffered state-sponsored persecution and attacks from mobs of Buddhists.

Thai authorities have been accused of turning a blind eye and complicity in the lucrative smuggling of Rohingya and Bangladeshi people seeking a new life in Malaysia, Indonesia or other countries.

In an apparent pointed message to Myanmar, which has left its 1.3 million population of Rohingya stateless, refusing them citizenship, a Thai Foreign Ministry spokesman said "countries of origin, transit and destination must work together to address the problem", adding that included tackling the "root causes".

Over the past three years tens of thousands of Rohingya have languished in squalid camps after attacks on their homes by Buddhist mobs in Myanmar.

The one-day summit in Bangkok will include officials from Australia, Indonesia, Malaysia, Bangladesh and Myanmar, which is also called Burma.

Thailand's junta leader Prayuth Chan-ocha has announced his country is considering setting up temporary camps to shelter Rohingya people where other countries and international organisations could assist in their repatriation.

Thai officials say they have found 74 camps used by human traffickers and rescued 213 illegal migrants and 63 trafficking victims in recent days.

Several Thai politicians and local officials have been arrested on human trafficking charges while 50 police are under investigation.

The UNHCR says as many as 920 people fleeing Myanmar died while crossing the Bay of Bengal in the six months to March from starvation, dehydration and beatings by boat crews amid a surge of human trafficking across South-east Asia.

A total of 53,000 people departed Myanmar and Bangladesh last year in what has become the largest movement of people in the region since the Vietnam War, the UNHCR says.