KETAPANG, Indonesia — A rehabilitation center in the jungles of Indonesian Borneo is taking in a growing number of orangutans as the country's worst wildfires in a decade sweep through the forest home of the critically endangered apes.

The intense wildfires, burning mainly on the islands of Borneo and Sumatra, began in July and could continue until November, the government has warned, aggravated by a severe El Niño weather pattern. El Niño, a periodic warming of sea surface temperatures in the Pacific, tends to lengthen dry spells across Southeast Asia, and past El Niño years have produced Indonesia's most severe fire seasons, when carbon-rich peat soils smolder underground for weeks and resist conventional firefighting. The 2015 fires, which coincided with a previous El Niño, went on to trigger one of the worst cross-border haze crises the region has recorded.

"I think so far this looks quite similar to the 2015 fires," said Karmele Llano Sanchez, co-founder of Yayasan Inisiasi Alam Rehabilitasi Indonesia (YIARI), who has worked in Indonesia for more than 20 years. "It's possible that in the months to come, they (the orangutans) will also be affected by not having enough to eat, not having enough room, enough habitat, and starvation."

The world's three species of orangutan are native only to Borneo and Sumatra. All three — the Bornean, Sumatran and Tapanuli orangutans — are classified as critically endangered by the International Union for Conservation of Nature, with shrinking, fragmented habitat among the chief threats to their survival.

YIARI, based in the Ketapang region of Indonesia's West Kalimantan province, is now caring for 90 orangutans after rescuing 49 since August, foundation spokesperson Heribertus Suciadi said. Over the past three years, the center had rescued only three to five animals a year, he added.

Hazardous Haze Across the Region

The fires and haze have shrouded parts of Borneo — which also includes the Malaysian states of Sarawak and Sabah — in hazardous smog that has spread to Singapore and the Malaysian peninsula, forcing some schools to close and driving a spike in respiratory illnesses.

Among the animals affected was Sampurna, an orangutan that Reuters encountered in August after it was found gasping for air and writhing on the ground in a palm oil plantation. The forestry ministry said last month that Sampurna died on August 30. Ms. Sanchez, who performed the necropsy on the great ape, said it died less than 12 hours after being rescued and had very dark lungs caused by respiratory problems, likening them to those of a heavy smoker.

From January to July this year, fires burned around 202,000 hectares (500,000 acres) in Indonesia. No official data for subsequent months has been released, but non-governmental organizations estimate that at least 600,000 hectares more have burned since. In 2015, fires burned nearly 960,000 hectares, forestry ministry data showed. How this year's toll compares with 2015 will become clearer once the government publishes official figures for the peak burning months.

"I think in general, what we see is that these orangutans that were surviving in these fragments of forest, and the forest fragments are becoming smaller and smaller and smaller," Ms. Sanchez said.

Rescue Missions

Once YIARI volunteers receive reports of orangutan sightings, they deploy a drone to determine which areas have been ravaged by the fires and whether orangutans are living there, Heribertus said. Equipped with tranquilizers, rescuers then drive for hours to the burned forest to find the apes, sometimes using a net to capture them before carrying them to safety. The foundation has more than 200 staff, including firefighters and veterinarians.

"Fires are occurring in many areas. As a result, the orangutans are being displaced into plantations," said Relly Susanto, a YIARI patroller.

On a recent August day at the YIARI center, orangutans swung through the trees and walked on vines as youngsters ate fruit beside their mothers. While there is no escape from the smoke even at YIARI, the center is not currently at risk from the fires themselves, and some of the rescued orangutans have been returned to the wild in safer locations.

YIARI currently has enough capacity to take in the orangutans, Ms. Sanchez said, but with the fires showing little sign of abating, she is unsure how long that will last.

"It's difficult to quantify how many orangutans will need to be rescued," she said. "One of our concerns is whether we'll have enough capacity to really get all of these rescued orangutans into the facility."

— Reuters