Thousands of migrants believed to be stranded at sea could die unless Southeast Asian governments act urgently to rescue them, the United Nations warned.
Nearly 2000 boat people from the two impoverished nations - many of them members of Myanmar's oppressed Rohingya minority - have swum ashore, been rescued or intercepted off Malaysia and Indonesia in recent days.
Many were thin, weak or in poor health after weeks at sea.
A migrant organisation said another vessel with about 350 on board was adrift somewhere near the Thai or Malaysian coast.
The Arakan Project, a group advocating for the rights of Muslim Rohingya said as many as 8000 other people may be adrift.
The group said it had spoken by phone with passengers aboard the vessel carrying 350 people, who said they were abandoned by their Thai human-traffickers.
"They told us they have had no food and water for the last three days. They have called for urgent rescue," said Chris Lewa, the group's founder.
Indonesia's navy, however, said earlier it had turned away one boat carrying hundreds of migrants from Myanmar and Bangladesh, whose fate remains uncertain.
Bangladesh, meanwhile, detained a trawler after it was cast adrift with 116 Malaysia-bound illegal migrants on board.
'Running for my life'
Ali Hussein, a 31-year-old Rohingya from Myanmar, is among more than 1000 people who swam ashore in Malaysia from their smuggling ships on Sunday and Monday.
Like thousands from the Rohingya community, he said he was "running for my life" from sectarian violence in Buddhist-majority Myanmar.
He and about 800 other people endured 43 days on an overcrowded vessel bound for Thailand as meagre food and water supplies dwindled to nothing.
Rohingya survivors of the route have previously told of harrowing sea passages in which passengers died of hunger or sickness or were beaten to death by smugglers, their bodies tossed overboard.
People-smugglers are believed to be dumping their human cargoes after being diverted from Thailand - a key stop on illicit migration routes - where authorities have cracked down on the trade.
The International Organization for Migration 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. They have boats and satellites," said Joe Lowry, a Bangkok-based spokesman for the IOM, a 157-member-state intergovernmental organisation.
Thailand launched its crackdown after discovering dozens of migrant corpses in secret jungle camps earlier this month.
On Tuesday, it said it would host a summit on 29 May to address "the unprecedented increase" in trafficking.
The United Nations refugee agency UNHCR says 25,000 people are believed to have embarked on the Southeast Asian route from January to March, double the previous year, and that an estimated 300 had died.
Vivian Tan, spokeswoman for the UNHCR in Bangkok, said traffickers may now be unwilling to land in Thailand and are holding people "captive" at sea or abandoning them.
But the IOM's deputy chief of mission in Indonesia Steven Hamilton said any regional effort to find stricken vessels would be daunting.
"It's like trying to find a needle in a stack of needles, not a needle in a haystack," he said.
"Those waters are dense with other boats. You're really not sure what you're looking for."