Indonesia stepped up aerial operations on Monday to extinguish forest fires raging on Sumatra island as Malaysia remained smothered by smog and Singapore enjoyed sunny skies thanks to favourable winds.

Sutopo Purwo Nugroho, spokesman for Indonesia's national disaster agency, said a fourth helicopter was deployed Monday for "water bombing" sorties in addition to two airplanes conducting cloud-seeding runs to induce rain over the parched island.

"We have carried out 14 water-bombings, dropping a total of 7,000 litres (1,820 gallons) of water onto the fires. To boost the operation, we have deployed an extra helicopter for water bombing today," he said in a text message to media.

Local police in Sumatra's Riau province said a landowner and a smallholder had been arrested for causing more than 400 hectares (988 acres) of peat land to catch fire. Firefighters earlier said they were having difficulty fighting fires on such soil.

Smog from Sumatra is a recurring problem during the June-September dry season, when plantations and smallholders allegedly set off fires to prepare land for cultivation despite a legal ban.

So far, attempts by Indonesia to induce rain have had little success.

A SilkAir plane from Singapore returned to the city-state early Monday after it failed to land at the airport in the Riau capital of Pekanbaru due to poor visibility from the thick smog, Indonesian news agency Antara reported.

An AirAsia flight from Bandung also scheduled to land at Pekanbaru was diverted to Kuala Lumpur, it added.

"The cloud-seeding technology is meant to speed up rainfall, but with few clouds, there's little we can do. The rain was more like a drizzle," Indonesian disaster agency official Agus Wibowo told AFP.

Officials in Singapore, which bore the brunt of the smog last week, warned against complacency, saying the situation could deteriorate again if monsoon winds carrying smoke and particulates from Sumatra changed direction.

Malaysia called on Indonesia on Monday to stop "finger-pointing" after its larger neighbour claimed several Malaysian companies are also responsible for forest fires.

"They are saying Malaysian companies are involved but Indonesian companies are also involved," Malaysia's natural resources and environment minister, G. Palanivel, told reporters.

Foreign Minister Anifah Aman added that whoever was responsible should be brought to book regardless of nationality and called it a problem for Southeast Asia, which suffered its worst smog outbreak in 1997-98 and a recurrence in 2006.

Much of Malaysia continued to wheeze under a shroud of smoke Monday with its southern half hit particularly hard. In the capital Kuala Lumpur, the pollution index neared the "very unhealthy" 200 level for the first time during the current outbreak.

Schools in Kuala Lumpur and several states were ordered to close and authorities advised parents to keep children indoors or make them wear face masks outside.

In one district close to Singapore, a state of emergency was declared after the Air Pollutant Index rating soared to 746 on Sunday, the country's highest recording since the 1997-1998 crisis.

Pollution levels in Malaysia's south eased Monday but generally worsened elsewhere, with the city of Port Dickson, which lies on the Malacca Strait across from Sumatra, hitting the "hazardous" 335 level.

Conditions in densely populated Singapore first began to improve from "harmful" on Saturday and the pollutant index at mid-afternoon Monday was between 50 and 100 -- within the "moderate" air-quality bracket.

Singapore Environment Minister Vivian Balakrishnan said on Sunday that "the improvement in the air quality is due to a change in the direction of the low-level winds over Singapore".

"However, we must remain prepared for further fluctuations depending on weather conditions," he added.

Organisers of an international conference in Singapore on reducing the threat of nuclear weapons, scheduled for Tuesday and Wednesday, said the event has been postponed due to the haze.

Singapore Foreign Minister K. Shanmugam as well as former US secretary of state George P. Shultz and former US defence chief William J. Perry were among the scheduled speakers at the 18-nation meeting.

"We are disappointed that we won't be able to host this historic gathering in Singapore this week," said Joan Rohlfin, president of the Nuclear Threat Initiative, one of the organisers.

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