Malaysian authorities say a cluster of abandoned jungle camps used by human traffickers contained 139 suspected graves as well as barbed-wire pens likely used to cage migrants, shedding more light on a regional trade that preyed on some of Southeast Asia's most desperate people.

National police chief Khalid Abu Bakar said Monday forensics experts were exhuming the suspected graves found at 28 vacated camps in the hilly jungle area on the border with Thailand where trafficking syndicates were known to operate.

"It is a very sad scene," Khalid told reporters at a police outpost in the town of Wang Kelian several kilometers (miles) from the camps, one of which appeared large enough to hold about 300 people. "I am shocked. We never expected this kind of cruelty."

At one forest camp, police found several parts of a decomposed body inside a wooden pen. The parts were placed into white bags and brought to Wang Kelian, and district police chief Rizani Ismail said they would be examined by forensics experts. Police said they would begin digging up other suspected graves a mounds of earth, covered with leaves and marked by sticks a on Tuesday.

"We have discovered 139 of what we believe to be graves," Khalid said. "We believe they are victims of human trafficking."

Prime Minister Najib Razak, in Tokyo on an official visit, vowed to find the perpetrators. "I am deeply concerned with graves found on Malaysian soil purportedly connected to people smuggling. We will find those responsible," he said on Twitter early Monday.

The finding in the northern Malaysian state of Perlis follows a similar discovery earlier this month by police in Thailand who unearthed dozens of bodies from shallow graves on the Thai side of the border. Thai police Maj. Gen. Puthichart Ekkachan said 36 bodies had been found there in seven abandoned camps.

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The discoveries have exposed hidden networks of jungle camps run by human smugglers, who have for years held countless desperate people captive while extorting ransoms from their families. Most of those who have fallen victim to the trafficking networks are members of Myanmar's persecuted Rohingya Muslim community or impoverished migrants from Bangladesh, part of a wave of people who have fled their homelands to reach countries like Malaysia, where they hope to find work or live freely.

As Southeast Asian governments have launched crackdowns in recent weeks amid intensified international pressure and media scrutiny, traffickers have abandoned camps on land and boats at sea to avoid arrest.

Khalid said at least two of the camps found in Malaysia appeared to have been abandoned within the past few weeks, based on the condition of items left behind such as vegetables, rice and almost new cooking utensils. Scattered personal possessions included a pink teddy bear and white children's sandals, he said, indicating the possible presence of children at the camp.

On three large whiteboards, police pasted dozens of pictures taken at the camps, which ranged in size, with the largest capable of holding about 300 people and smaller ones just a few dozen. Some of the pictures showed large, crudely built pens made from wooden sticks and barbed wire that were believed to be used as cages.

"We think the migrants were imprisoned in these wooden pens," Khalid said. "They were not allowed to move freely and traffickers kept watch at sentry posts."

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Other items left behind included a rifle pouch, used bullets and bullet casings, he said. Police also found white muslin cloth, used by Muslims in Malaysia to wrap dead bodies, and a wooden stretcher believed to have carried bodies.

The findings were the result of an operation Malaysian authorities conducted from May 11-23, during which they combed the steep jungle area along 50 kilometers (30 miles) of the border with Thailand. Due to the difficult terrain, Khalid said the investigation and forensics analysis could take a few weeks.

Human rights groups and activists say the area on the Thai-Malaysia border has been used for years to smuggle migrants and refugees, including Rohingya Muslims, a long-persecuted minority in predominantly Buddhist Myanmar.

In many cases, they pay human smugglers thousands of dollars for passage, but are instead held for weeks or months, while traffickers extort more money from their families back home. Rights groups say some have been beaten to death, and The Associated Press has documented other cases in which people have been enslaved on fishing boats.

Since May 10 alone, more than 3,600 people a about half of them from Bangladesh and half Rohingya from Myanmar a have landed ashore in Indonesia, Malaysia and Thailand. Thousands more are believed to be trapped at sea in boats abandoned by their captains.

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Malaysia and Indonesia announced last week that they would provide temporary shelter for up to one year for migrants recently found or still stranded at sea. The U.S. has said it will settle some of them permanently.

The Rohingya, numbering around 1.3 million in Myanmar, have been called one of the most persecuted minorities in the world. Long denied basic rights, they have been driven from their homes in mob attacks in Myanmar's Rakhine state several times since 2012.

More than 140,000 were displaced and are now living under apartheid-like conditions in crowded camps. More than 100,000 others have fled by sea.