Indonesia vowed Wednesday to "wage war" against illegal land burning after officials were detained and faced death threats from a mob allegedly trying to stop them investigating smog-belching fires.

The incident, which saw seven investigators held by a 100-strong gang allegedly hired by a palm oil company, has highlighted the difficulty faced in tackling the raging blazes that cloak Southeast Asia with haze every year.

The fires and subsequent smog occur annually to varying degrees on Sumatra island and the Indonesian part of Borneo during the dry season, and are started to cheaply clear land for palm oil and pulpwood plantations.

But 2015's were the worst for years and saw large parts of Indonesia, Malaysia and Singapore cloaked in choking smog for weeks. They have not been as serious so far this year.

The seven-strong team were detained by the mob in Riau province, on Sumatra island, on Friday after taking photos of land that had allegedly been cleared with fire by a company called Andika Permata Sawit Lestari (APSL).

The mob -- suspected to have been hired by APSL -- threatened to beat them, kill them and dump their bodies in a nearby river. They were finally released unharmed after 12 hours when police intervened.

Indonesia's Environment Minister Siti Nurbaya Bakar has condemned the incident, saying it highlights how companies form murky alliances with local communities to burn land and protect their plantations.

After meeting the head of the national police on Wednesday, Bakar said: "I have the police chief's backing to wage war against forest and land fires."

Police chief Tito Karnavian said a joint police and ministry team would carry out investigations in Riau.

Under fierce pressure from its neighbours, Indonesia has pledged to take more action and has arrested over 460 people so far in 2016 over forest fires, more than double the number detained last year.

But activists say the latest case highlights how under-resourced officials and security forces are often no match for the massive companies that are accused of setting the illegal fires.