Malaysia has begun exhuming bodies suspected to be migrants buried in 139 grave sites close to the Thai border.
Authorities believe the migrants were held for ransom in jungle camps by gangs of human traffickers.
Thousands of migrants from Bangladesh and Burma have left for Thailand, Malaysia and Indonesia in recent weeks.
Thailand has offered a "floating naval base" to help migrants stranded at sea but said only those needing serious medical treatment will be taken ashore.
Meanwhile, authorities in Burma have charged 20 people with offences related to human trafficking.
They were arrested from a vessel carrying 200 migrants from Bangladesh that was rescued last week.
Yesterday, Malaysian police teams began sifting through a series of camps found in the jungle near the country's border with Thailand.
Some migrants were apparently held in "human cages" made of wood and barbed wire at the camps, which officials believe were only abandoned recently. Malaysian national police chief Khalid Abu Bakar said there were signs that torture had been used, but he did not elaborate.
"We were shocked by the cruelty," Mr Abu Bakar told reporters.
Traffickers The discovery of the camps in Malaysia followed the uncovering of similar graves on the Thai side of the border at the beginning of May.
After those graves were found, Thailand began cracking down on the routes used by traffickers to move Bangladeshi migrants and Rohingya Muslims from Burma through its territory.
Meanwhile, migrants found at sea have been given temporary shelter in Malaysia and Indonesia - but not in Thailand
The migrants, who want to reach Malaysia, were instead being sent by sea, but thousands were left drifting as no country wanted to take them in.
The UN's refugee agency says about 3,000 people have now reached land, but estimates another 2,600 may still be out at sea.