The United Nations human rights office has called on Thailand to speed up an investigation into the disappearance of an ethnic minority environmental activist.

Pholachi "Billy" Rakchongcharoen was detained in Thailand's Kaeng Krachan National Park last April for allegedly possessing illegal wild honey but officials say he was released shortly. He has not been seen since.

The UN's Human Rights Office for Southeast Asia said Thursday the Thai government should strengthen its efforts to carry out a transparent probe into the case.

Before his disappearance, Billy was helping ethnic Karen villagers and activists prepare a lawsuit accusing park officials of burning and destroying the homes and property of families in the area.

Thai police say they are working on the case but added there has been no progress.