Eight orangutans have been released back into the wild in Borneo after they were taken into captivity when their habitat was threatened by wildfires over the summer.

Three mothers, their three daughters and two adult males were freed in a reserve in the west of the island on Friday, International Animal Rescue Indonesia (Yiari) and the forestry ministry said.

Indonesian authorities rescued 46 orangutans from fires in August and September. Wildfires in the region are usual for the country's dry season but intensified this year.

The apes are critically endangered. Around 104,000 orangutans remain in Borneo and Sumatra, down from more than 230,000 a century ago, according to conservation group WWF.

While Indonesia is among many countries to grapple with wildfires each summer, this year's were more intense due to the El Niño ocean warming phenomenon, which makes weather patterns more volatile and intense.

Yiari CEO Karmele Llano Sanchez said relocating the eight orangutans to the reserve was a "necessary step" when their habitats became unsafe.

Among them was Mama Nopi and her offspring Nopi, Naufal and Nopan, who had been surviving in a coconut and rubber plantation before their rescue in early September, Yiari said.

Fires had incinerated the surrounding forestry, restricting their access to food and bringing them closer to humans.

Mama Lifa and her child Lifa were found in a patch of acacia trees entirely surrounded by oil palm plantations.

The degradation of their habitat by wildfires as well as a lack of food sources prompted their rescue in mid-September.

Mama Man and her baby, Man, had been monitored by animal rescuers since June, when they were found living in a plantation area adjacent to a small patch of forest.

Fires that spread near to the site restricted their roaming area further, leaving them without a safe route to other forestry. The threat of exposure to the blazes led to them being evacuated in early September, Yiara said.

The reserve these orangutans were released into was chosen because it there is food and water available and it is connected to other forested areas.

Yiari's Argitoe Ranting said: "We want to ensure that rescued orangutans are not simply moved from one place to another, but actually have a chance to survive in their natural habitat."

Last month, the International Union for Conservation of Nature warned that wildfires were burning in habitats containing 52% of Borneo's orangutan population.

It says a large portion of the ape's habitat will become inhospitable to them if climate change continues at its current pace.

This year's El Niño has become the strongest on record and is expected to have a knock-on effect on the climate into next year.

Experts have also pointed to land-clearing practices used to make room for plantations as another factor facilitating the spread of wildfires in Indonesia, with naturally fire-resistant forests replaced by less dense and shaded vegetation.

That the orangutan's original habitats had been burned and fragmented to the point they could no longer sustain the primates was the main driving factor behind relocating them, Yiara said.