Malaysia ordered school closures in Kuala Lumpur and neighboring states as worsening haze from Indonesian forest and agricultural fires enveloped the capital in a shroud of smoke.

Cloud-seeding aircraft were meanwhile being deployed in the hope of triggering rain to help clear the smoke, Malaysian authorities said.

Air quality in Malaysia and Singapore has steadily deteriorated over the past two weeks due to the smog from nearby regions of Indonesia, which on Monday declared a state of emergency in a particularly hard-hit province.

On Tuesday Malaysia?s education ministry ordered schools closed in Kuala Lumpur, three adjacent states and the nearby administrative capital of Putrajaya, where air pollution indices were in the upper range of ?unhealthy? and nearing ?very unhealthy?.

More than half of the country?s 52 monitoring stations registered ?unhealthy? air on Tuesday.

A cloud-seeding plane carrying chemicals meant to trigger rain would take off on 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 was planned for three days but would 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.

Smoke from fires is an annual problem during the dry season in Indonesia, where vast tracts of land are cleared on the huge islands of Sumatra and Borneo using illegal slash-and-burn methods used to clear land for palm oil and pulp and paper plantations.

Indonesia has declared a state of emergency in Sumatra?s Riau province. One thousand military personnel were dispatched there while 600 were sent to South Sumatra province to help local authorities fight fires, Indonesia?s disaster agency said. They join over 1,000 soldiers sent to Sumatra last week.

President Joko Widodo said he had ordered law enforcement agencies to ?take firm legal action against parties responsible for the forest fires?.

Tens of thousands of people in smoke-choked regions of Sumatra and Borneo have fallen ill, while air travel there – as well as in parts of Malaysia – has been hit by flight delays or cancellations due to poor visibility.

Malaysian authorities in several areas have distributed free face masks, while the marine and aviation sectors have been advised to go on high alert due to the worsening visibility.

Authorities advised people with respiratory conditions to wear face masks outdoors and for all citizens to limit unnecessary outdoor activity.

Indonesia has come under pressure to control the annual smog outbreaks but joint regional efforts have failed to find a lasting solution.