Villagers on Doi Pui mountain in Chiang Mai have apologised for allowing volunteers to "clean up" a memorial for the late King Bhumibol Adulyadej (Rama IX), resulting in 15 large trees being cut down.

The apology is deemed insufficient and the government will take legal action against those held responsible for felling the trees, which were inside a national park.

Kritsiam Kongsatree, director of the Protected Area Regional Office 16, said on Tuesday that the apology was conveyed by Methaphan Phuchakrissadapha, the head of tambon Suthep, on behalf of the villagers living on Doi Pui.

They had approved a landscape improvement proposal made by a group of volunteers but had not expected them to chop down any trees, Mr Methaphan told him.

Fifteen large trees were felled at the memorial, which was built by Doi Pui villagers in honour of the late king for his frequent visits to the mountain and royal projects aimed at replacing illegal poppy crops with legal commercial crops.

The memorial opened in 2007 and is inside Doi Suthep-Pui National Park.

No volunteer group has admitted responsibility for the tree chopping. However, a Ban Hmong Doi Pui Facebook post on Oct 9 showed the Suthep Group for Sustainability and some villagers cleaning up the place. One had a chain saw on the ground beside him.

The investigation was continuing.