KUALA LUMPUR, Malaysia (AP) — A layer of heavy haze forced Malaysian authorities to shut schools Tuesday in three states and two key cities as aircraft were to begin cloud-seeding operations to induce rain to help clear the air.

The thick, dirty white haze blanketed many parts of Malaysia — an annual phenomenon that is mostly caused by the burning of forests in Indonesia to clear land for farming. Some 34 of 52 air quality stations recorded unhealthy air levels early Tuesday.

A plane equipped with chemicals it aims to release in the air that will help clouds produce rain was due to take off later Tuesday to cover Kuala Lumpur and surrounding areas, said Maznorizan Mohamad, a meteorological department senior official. A second aircraft was scheduled to fly over Kuching in Sarawak state on Borneo island.

She said the cloud-seeding is planned for three days but will depend on cloud availability and weather conditions. The inter-monsoon season is expected to start in late September, bringing more rain over peninsular Malaysia to clear up the haze.

"It will bring temporary relief but whatever it is, we have to address the source of the problem," she said.

Malaysia's education ministry earlier ordered schools in Kuala Lumpur, government administrative capital Putrajaya as well as Selangor, Negeri Sembilan and Malacca states to close Tuesday.

Indonesia's Riau province has declared an emergency that shut schools and set up health posts to treat those suffering respiratory problems after its air pollutant index hit extremely dangerous levels.

In Singapore, air pollution reached very unhealthy levels. Organizers of this weekend's annual Formula One night race are keeping a close watch on the situation to decide whether it will be safe to race. The Singapore Grand Prix has always been held at this time of year, and while there has been some haze in past years due to the forest fires, it has never prevented the race from going ahead.

Indonesia's government has dispatched planes and helicopters for cloud seeding and water dropping, along with more than 1,000 soldiers sent to Sumatra island to help extinguish the fires.