Thousands of refugees from Bangladesh and Myanmar are stranded at sea close to Thailand, according to an international migration agency.
The International Organization for Migration (IOM) told the BBC a Thai crackdown on recent arrivals meant many smugglers were now reluctant to land.
As many as 8,000 people are believed to be stuck on boats, the IOM said.
In the past two days more than 2,000 have arrived in Malaysia or Indonesia after being rescued or swimming ashore.
Jeff Labovitz, head of mission for IOM Asia Pacific, told the BBC that the discovery last week of dozens of human remains in abandoned camps in the south of Thailand had prompted a police crackdown and therefore people smugglers were holding their boats at sea.
Bangladeshi migrants, and ethnic Rohingyas who face persecution in Myanmar, are normally brought by people smugglers to Thailand where they are effectively held ransom until they can raise money for their onward journey to Malaysia and beyond.
“Boats seem to have stopped coming – but some are en route and some are waiting to off-load. Where they used to process people on land, for final payments, they are now doing this off-shore – so more people are being held off-shore. Now things have become so hot, there’s nowhere for them to go,” Mr Labovitz said.
He said that an estimate by the Arakan Project – which monitors the movements of Rohingyas – that 8,000 people are stranded at sea could not be verified, but seemed credible.
Malaysian officials said on Monday that 1,018 Bangladeshi and Rohingya refugees had landed illegally on Langkawi island, apparently abandoned by people smugglers who were transporting them to Thailand.
Indonesian authorities rescued nearly 600 migrants stranded off the coast of Aceh on Sunday and more than 400 others early on Monday.
A report by the UN refugee agency UNHCR said on Friday that 25,000 migrants boarded people smugglers’ boats from Myanmar and Bangladesh in 2015’s first quarter – about double the number who left over the same period in 2014. It said about 300 people died at sea from January to March this year.
It added that the increase in migration from the area has been attributed to a variety of factors, including political developments in Myanmar, and wives and children wishing to join the men who have already migrated.
Three Thais and a Burmese national have been arrested on suspicion of human trafficking after last week’s discovery of shallow graves in abandoned camps in southern Thailand.