Indonesia warned Friday that haze from forest fires was floating over a key waterway towards its neighbours, and that the number of blazes was rising.

The fires and resulting smog are an annual dry season problem in the archipelago, when blazes are started illegally to quickly and cheaply clear land, typically to make way for palm oil and pulpwood plantations.

But last year's haze outbreak was among the worst in memory, shrouding Malaysia, Singapore and parts of Thailand in acrid smoke. The crisis forced school closures and caused thousands to fall sick across the region.

While this year's fires have yet to reach the levels of 2015, the number has been rising in recent weeks as Indonesia heads towards its peak dry season in September.

Disaster agency spokesman Sutopo Purwo Nugroho warned that smoke had Thursday started floating across the Malacca Strait, which runs between Indonesia, Malaysia and Singapore.

"Smoke from forest and land fires in Riau (province) has started to enter the Malacca Strait," he tweeted.

"Let's prevent and put out the fires."

Riau, on western Sumatra island, is a major centre of the palm oil and pulpwood industry, and many fires occur there every year.

He also said the number of "hotspots" detected by satellites -- areas of intense heat that are either already on fire or vulnerable to going up in flames -- had increased in West Kalimantan province, on Indonesia's part of Borneo island.

A total of 158 hotspots were detected in the province on Friday, up from 106 a day earlier.

The governor of the province, a centre of the palm oil industry, had asked the disaster agency to provide helicopters for water-bombing and "cloud-seeding", or chemically inducing rain, said Nugroho.

Indonesia has faced intense criticism from its neighbours and the international community over its failure to halt the annual smog outbreaks.

Jakarta has promised tougher action. It has announced a plan to stop granting new land for palm oil plantations, and established an agency to restore millions of hectares of carbon-rich peatlands susceptible to fires.