JAKARTA/PEKANBARU - Indonesia has arrested 230 people suspected of setting fire to forests in Sumatra and Kalimantan, which has resulted in thick, acrid smoke spreading across South-east Asia, as the country carried out cloud-seeding operations in three provinces on Wednesday (Sept 18).

Speaking to reporters, Indonesian national police spokesman Dedi Prasetyo on Wednesday also named an operational director of a South Sumatra-based company a suspect, adding that somebody in such a position has the greatest responsibility to take mitigation measures in the land management.

"There should be maximum efforts to prevent the land under (company's) concessions from fires. Companies are mostly negligent," he told reporters.

Under the current environment law, the suspects could be prosecuted and face a maximum 10-year prison sentence for setting fires to clear land.

The forest fires are often blamed on small holders who light up the forest during dry weather to quickly clear land so that they can raise crops such as oil palm, or by large plantation companies to plant oil palm and pulpwood trees for paper.

Indonesia said last week that it has sealed off plantations operated by 42 companies, including subsidiaries of four Malaysian groups and one Singapore-affiliated firm, after fires were detected in their concessions.

Meanwhile, Indonesia's National Disaster Management Agency (BNPB) carried out cloud-seeding operations on Wednesday in Riau, North Sumatra and Central Kalimantan.

After the operation in Riau, rainfall of medium intensity fell for around 30 minutes in Dumai, BNPB acting spokesman Agus Wibowo said in a statement on Thursday.

On Thursday, two aircraft were deployed in Pekanbaru, Riau province, with the discovery of dense clouds that could be seeded to induce rains.

One aircraft was heading south-east of Pekanbaru, and the other heading north-northeast, said Mr Samba Wirahma, a cloud seeding field coordinator in Pekanbaru, with the Agency for the Assessment and Application of Technology (BPPT). Heavy rain fell in Meranti regency on Thursday.

The cloud seeding is deemed important to douse fires, particularly in peatland areas, as that may "take time" and "need a lot of water", Mr Jim Gafur, the head of Riau BNPB told The Straits Times last Sunday. Peat fires are harder to extinguish, as they can continue burning underground for weeks, especially in deep peat areas, that could result in thick and acrid haze being produced.

Indonesia said this week that it was deploying thousands more officers to battle the forest fires on two of the country's five main islands - Sumatra and Kalimantan - bringing the total number from 9,072 to 29,039.

From January to August, tracts in the world's third-biggest tropical rainforest consumed by fires totalled 328,724ha, of which 27.3 per cent were peatlands, according to BNPB. The two biggest tropical rainforests are in the Amazon and Congo basins.

The BNPB emergency response team has used at least 52 helicopters for water bombing every day since the forest fires started spreading about a month ago.

In a separate statement, Indonesia's weather forecast agency BMKG said heavy rainfall could come in the next seven days in Aceh, North Sumatra, North Kalimantan (northern coastal areas) and Papua.

The agency also said that the number of hot spots across Asean had declined in the Sept 16 to 18 period to 3,302 from 3,854 in the Sept 13 to 15 period.

On Sept 14, there were still 1,080 hot spots in Indonesia. But from Sept 15 to 17, the number of hot spots had fallen, although the figure was up again in Sept 18.

Transboundary haze from Sumatra had spread into Peninsular Malaysia, BMKG said.

Haze also spread from West Kalimantan into Sarawak. This was because the wind in Riau blew from south-east to north-east, while wind in West Kalimantan headed to the north. Meanwhile, the Philippines' Environment Management Bureau said on Wednesday that the Indonesian haze had reached Cebu City in central Philippines.