Malaysia has begun exhuming bodies suspected to be migrants buried in mass grave sites found by the Thai border.Authorities believe migrants were held for ransom in jungle camps, and those who died were buried in 139 graves.
Thousands of migrants have left Myanmar and Bangladesh in recent weeks on boats and overland through Thailand and Malaysia. Some have ended up in the hands of human traffickers.
Thailand has meanwhile offered a "floating naval base" to help migrants.
Officials believe the camps were only abandoned recently, following a crackdown by Thailand on human traffickers after similar mass graves were found in its southern Songkhla province.
Some migrants were apparently held at the camps in "human cages" made of wood and barbed wire.
Malaysia's 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," said Mr Khalid.
Referring to the "cages", he told reporters: "We think the migrants were imprisoned in these wooden pens...They were not allowed to move freely and traffickers kept watch at sentry posts."
Earlier this month, Thailand began cracking down on the routes used by traffickers to move Bangladeshi migrants and Rohingya Muslims from Myanmar through its territory.
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.
Malaysia and Indonesia this week agreed to stop rejecting boats and to provide temporary shelter to the migrants.
But while Thailand said it would stop towing away boats from its territory, it said would not take in migrants.
On Monday, facing international pressure to do more, Thailand's Prime Minister Prayuth Chan-ocha said the country would establish a "floating naval base" to help migrants.
A navy ship with surveillance aircraft will give supplies and medical treatment to migrants stranded in the Andaman Sea.
But only people with serious medical needs will be taken to land, where they will be considered illegal arrivals.
"If anyone is injured or sick, they can be treated at hospitals in Thailand but will face illegal entry charges - it is up to them to decide," he said.