Seven fire investigators from Indonesia's Environment and Forestry Ministry were illegally held for several hours by a mob of men who threatened to burn them alive and dump their bodies in the river.

Last Friday's incident took place after the officers had gone to inspect a plantation in Riau province where illegal burning was detected.

The officials were released only on Saturday morning after they gave in to demands by the group to delete photo and video evidence collected during their survey of the concession in Rokan Hulu regency.

The ministry said its officials were there to follow up on hot spots detected by satellites when haze blanketed the area recently.

Environment and Forestry Minister Siti Nurbaya Bakar, in a statement yesterday, condemned the actions of the 100 or so men, who are believed to be working for the palm oil plantation firm, Andika Permata Sawit Lestari (APSL).

Ms Siti said her ministry's "main priority" now is to launch an investigation into ASPL, which has no permission to cultivate land in Rokan Hulu. She said other material evidence from a drone camera, which the mob did not find, showed some 2,000ha of land burnt by APSL workers pretending to be farmers.

"As far as we can see, the land, which was formerly a peat swamp forest, is now a palm oil plantation," said Ms Siti, adding her team also found ashes from "intentional fire" as well as several points where smoke was rising from the ground.

"We found evidence from the field that showed thousands of hectares of palm oil trees burnt in the production forest where cultivation is not permitted," she said.

"In other words, the palm oil plantation in the area is illegal."

APSL could not be reached for comment yesterday.

According to the ministry, the seven officials had arrived in Pekanbaru last Monday and travelled to Rokan Hulu over the next few days to carry out their inspection.

On their final visit on Friday, they were about to leave the plantation on a pontoon when they were stopped by the men and taken to another location.

The police arrived at about midnight to negotiate the officers' release but the group, which had grown to around 100, demanded a meeting with Ms Siti. Negotiations went on for another two hours before the investigators were released after they agreed to destroy photo and video evidence.

Friday's incident comes as the Indonesian authorities' crackdown on individuals and companies that use the slash-and-burn method to clear land has begun to bear fruit, with hundreds of arrests made.