BANGKOK (AFP) - Thailand has deployed a helicopter carrier in its waters to serve as a temporary medical and processing centre for migrants found adrift, its prime minister said on Monday.

The Thai carrier will act as a "floating base with medics and police" on board, Prime Minister Prayut Chan-O-Cha told reporters. "If any boats (with migrants) are found, the navy will lead it to this large navy vessel for assistance," he said, adding Thailand will then "facilitate" their passage to "temporary shelters in Malaysia and Indonesia".

"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."

A navy official told AFP the vessel was a helicopter carrier.

The move came as the United States offered to launch reconnaissance flights over the Bay of Bengal to locate any remaining vessels, with thousands of Bangladeshi and Myanmar boat people still believed to be stranded at sea.

Thailand is hosting a May 29 regional meeting on a crisis that has seen more than 3,500 Bangladeshi economic migrants and stateless Rohingya Muslims from Myanmar arrive on Thai, Malaysian and Indonesian soil.

All three countries, as well as Myanmar and Bangladesh, have come under international pressure either to provide immediate humanitarian relief to the migrants or address the root causes of their journeys.

The UN estimates 2,000 migrants are still at sea on poorly supplied boats after a May 1 crackdown on human trafficking in Thailand paralysed the route south, apparently leading people-smugglers to abandon ships crammed with human cargo.

The US embassy in Bangkok said America was "actively engaging with the governments of the region to obtain their support and permissions" for flights over the seas to spot stricken migrant boats.

But Mr Prayut, a former army chief who leads the ruling junta, said any US missions would have to be conducted with Thai forces.

"We have started our operation, if they (the US) want to join, they can... (but) they cannot control the operation, we have to have sovereignty over both land and sea."

On Monday, Malaysian police said they had found 139 grave sites and 28 abandoned detention camps used by people-smugglers in a border area close to where Thailand's discovery of mass graves in May sparked the unravelling of trafficking routes.