The wife of a Thai activist feared dead after he went missing last month said Tuesday police have failed to properly investigate his case, which threatens to to add to the country's dismal record of unsolved disappearances.

The United Nations has recorded at least 82 open cases of enforced disappearance in Thailand dating back to the 1980s.

The latest to vanish is Den Kamlae. The 65-year-old activist had been leading a long-running battle to secure a land deed for his community in northeastern Chaiyaphum province, which authorities accuse of encroaching on protected forest.

His wife and other villagers are convinced his disappearance is linked to his activism.

"There is no other reason why he could be missing," said Supap Kamlae, who has travelled to Bangkok to petition government agencies and rights groups for help.

But the chief police officer behind the investigation dismissed the theory that Den had been murdered.

"We suspect he may have suffered heat stroke or was attacked by wild animals," Phan Yosrungrueang, from Huay Yang district police station, told AFP.

Supap last saw her husband on 16 April, when he headed off to forage for bamboo shoots in the forest near their home.

Den's friends and relatives told AFP they mounted their own search after police hastily wrapped up their hunt for him.

But they have not found his body despite scouring the forest every day for weeks -- including using diving equipment to search ponds.

"We want help. We are poor and we need help...We feel powerless," Supap told AFP.

Pornpen Khongkachonkiet, the director of the Cross Cultural Foundation, an NGO publicising Den's case, said Thailand's poor face a steep hill to justice in a legal system biased towards the rich and influential.

"We don't have equality in the law," she told AFP, adding that local police often do not take reports of missing people seriously.

The community's plea for help follows the two-year anniversary of the disappearance of another land activist.

Por Cha Lee Rakcharoen, an ethnic Karen activist known as Billy, vanished in 2014 after he was ostensibly arrested by officials in Kaeng Krachan National Park for carrying illegal honey.

Two years later his disappearance remains shrouded in mystery, despite his family's efforts to prosecute park authorities they believe killed the activist over his efforts to sue them for allegedly torching the homes of 20 families in the park in 2011.

In January the UN called on the Thai government to do more to uncover the fates of the country's disappeared.

It also criticised the recent acquittal of five police officers accused of abducting Somchai Neelapaijit, a Muslim human rights lawyer who has been missing for 12 years.