BANGKOK: Thai police said on Tuesday they had "shown sincerity" and wrapped up the country's biggest investigation into human trafficking, as rights groups questioned whether they had even scratched the surface.

Thailand began a crackdown on trafficking networks and suspected camps hidden deep in its jungle-carpeted mountains last month following the discovery of more than 30 bodies buried in camps in the south.

Police have arrested 56 suspects - including politicians, police, government officials, businessmen and an army general - and issued arrest warrants for 63.

Aek Angsananont, police deputy commissioner-general, called the probe "the biggest human trafficking investigation in Thailand's history".

Around 1,000 police officers, many of them based in southern Thailand, took part in the investigation, Aek said, without elaborating on any ongoing or future probes.

Police have sent 19 cases filled with more than 100,000 document sheets to the Office of the Attorney General which will have until July 24 to decide whether to file charges, Aek said.

"This government has shown its sincerity in solving this problem by seriously tackling human trafficking and by dealing with those involved," Aek told Reuters.

But rights groups, including the New York-based Human Rights Watch, said the investigation would not put an end to networks operating in the region.

"It is highly likely that if this investigation turns out to just be window dressing to defuse international pressure, then it will have no impact," Sunai Phasuk, Thailand researcher at Human Rights Watch, told Reuters.

"It will just put them under ground for the time being and then resurface again."

The crackdown in Thailand, a traditional transit point, made it too risky for traffickers to land migrants, Bangladeshis and Muslim Rohingya from Myanmar, prompting them to abandon thousands at sea with scant supplies of food and water.

The United Nations estimates 1,200 people are still at sea or unaccounted for while more than 3,000 have landed since May in Thailand, Malaysia and Indonesia.

Last year, Thailand was downgraded to the U.S. State Department's lowest category - or Tier 3 - in its annual Trafficking in Persons Report, which assess how governments around the world have performed in fighting human trafficking.

The next TIP report is expected in mid-July.

"We hope the United States will consider to put our country, which is in Tier 3, to a better tier," Aek told Reuters.

Police in neighbouring Malaysia last month discovered 139 graves in jungle camps used by suspected smugglers and traffickers. They say some 12 officers are being investigated.