AFP/ Padang Besar

Thai authorities on Saturday resumed the excavation of a mass grave site in a remote patch of jungle where migrants believed to be from Myanmar and Bangladesh were held for months by people smugglers in appalling conditions.

Eight bodies have been recovered so far from the abandoned camp in Sadao district, in Songkhla province, a few hundred metres (yards) from the border with Malaysia.

The grim discovery has once again exposed Thailand's central role in a regional human trafficking trade.

Four shallow holes in the dirt a short walk behind the camp mark where the bodies were retrieved, an AFP reporter at the scene said, while white ribbons tied to a few dozen bamboo poles delineated suspected new graves.

The cause of the migrants' deaths is not yet clear, but details emerged Saturday of the conditions they endured, in what Thailand's police chief has described as a "virtual prison camp".

Doctors treating the two sole survivors -- men aged 25 and 35 -- told AFP their patients were suffering from a range of ailments.

"Both are malnourished, have scabies and lice," doctor Kwanwilai Chotpitchayanku told AFP at Padang Besar hospital.

"The older man could not walk, he had to be carried off the mountain. He hadn't eaten anything for two days before he was found. He told the translator he had a fever in the jungle for two months."

Doctors said the men had not been fully identified but were from either Bangladesh or Myanmar.

Both were rigged to IV drips and appeared frail as they lay in their ward beds.

The border zone with Malaysia is criss-crossed by trafficking trails and is notorious for its network of secret camps where smuggled migrants are held, usually against their will, until relatives pay up hefty ransoms.

- Trafficking 'out of control' -

Rights groups say the camp, which is a steep, slippery 40-minute hike from the nearest road, is likely to be just one of dozens in the area as the rewards of trafficking continue to outweigh the risks of being caught.

A rescue worker told AFP that four of the dead were "skeletons" while the fifth died just a few days ago, seeming to indicate the camp had been in existence for some time.

Tens of thousands of migrants from Myanmar, mainly from the Rohingya Muslim minority but also increasingly from Bangladesh, make the dangerous sea crossing to southern Thailand, a well-worn trafficking route often on the way south to Malaysia and beyond.

The exodus of Rohingya -- described by the UN as one of the world's most persecuted minorities -- has followed deadly communal unrest in western Myanmar's Rakhine state since 2012.

Thailand says it is cracking down on the trafficking networks on its soil after revelations that government officers, police and navy officials have been involved in the lucrative trade in humans fleeing poverty and persecution.

"We will go after the people responsible (for the grave site) no matter how powerful they may be," General Aek Angsananont, national police deputy commissioner, told reporters in Padang Besar.

"We care about our image, when people say we're not doing anything about it, it's not true it's a national agenda."

In June the United States dumped Thailand to the bottom, or "Tier 3", of its list of countries accused of failing to tackle modern-day slavery.

Activists say traffickers are changing their tactics as the crackdown bites and are also holding thousands of migrants at sea for endless weeks awaiting payment before releasing them.

Thailand's human trafficking problem is "out of control", according to Brad Adams, Asia director of Human Rights Watch.

"The finding of a mass grave at a trafficking camp sadly comes as little surprise," he said, urging the UN to join the probe to bring those responsible to justice.