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The number of dry season fires burning across Indonesia has jumped to more than 500 and a sixth province has declared a state of emergency, the disaster mitigation agency said Tuesday.

Spokesman Sutopo Purwo Nugroho said satellite images show 538 fires across 23 provinces, up from about 160 early in the month.

Plantation companies and villagers often set the fires deliberately to clear land for planting. They spread easily because a widespread practice in Indonesia of draining swampy peatlands for palm oil and pulp wood plantations has made swathes of land highly combustible.

Nugroho said Central Kalimantan was the latest province to declare a state of emergency. Last month five provinces — Riau, Jambi, South Sumatra, West Kalimantan, and South Kalimantan — declared emergencies in anticipation of a worsening of the fires and to enable measures to mitigate the choking smoke that peatlands generate when burned.

He said the agency has deployed 21 helicopters to help extinguish fires in Sumatra and Kalimantan as well as a Casa 212 aircraft to induce rain in Sumatra.

More than one-third of the fires are in West Kalimantan on the Indonesian part of Borneo and one quarter are in the easternmost Indonesian region of Papua. Typically the fires have affected Sumatra and Kalimantan the most, but plantation companies have found new frontiers in Papua's wilderness, exposing it to greater fire risk.