Endangered orangutans are falling victim to a devastating haze crisis that has left them sick, malnourished and severely traumatised as fires rage through Indonesia's forests, reducing their habitat to a charred wasteland.

Rescuers at a centre for the great apes on Borneo island are considering an unprecedented mass evacuation of the hundreds in their care, and have deployed teams on hazardous missions to search for stricken animals in the wild.

At the Nyaru Menteng centre in Kalimantan, sixteen baby orangutans have been put into isolation, suffering infections from prolonged exposure to the thick, yellow smoke suffocating Indonesia's half of Borneo island.

A devoted carer tries to entertain the youngsters with toys and games as the infants recover from high fevers and serious coughs.

In another enclosure, several orangutans lie about listlessly, too exhausted to move after days hunting for food and water as fires relentlessly encroached on their forest homelands, forcing them to flee.

Others swing repeatedly from bar to bar, occasionally pausing to make a distinct smacking with their lips -- a sound that makes their carers anxious.

"That's called a quick kiss," said Hermansyah, a carer at the centre, who like many Indonesians goes by one name.

"When they make this gesture, it means they are under tremendous stress," he told AFP.

- Massive destruction -

The fires from slash-and-burn farming -- a method to quickly and cheaply clear land for new plantations -- have so far destroyed 1.7 million hectares (4.2 million acres) in Kalimantan and neighbouring Sumatra.

The vast plumes of smoke have drifted over large expanses of Southeast Asia, sickening countless people, disrupting transport, schools and business, and drawing outrage from neighbouring governments.

Despite being a near annual occurrence, the toxic cloud is on track to become the worst on record, and staff at the centre say the intensity of the smoke and flames at ground zero has never been seen before.

As the situation reached crisis point in recent months, with suffocating yellow smoke descending on the region, staff began receiving alarming reports of stricken apes and deployed emergency teams.

A team made up of a veterinarian, a professional climber and a technician armed with sedative darts has been trekking into the burning forest on hazardous missions, battling flames and smoke.

"Many times the visibility was as low as 30 metres, and we felt chest tightness and headaches," Hermansyah said.

"We believe these orangutans were also experiencing the same suffering."

Many orangutans rescued from the fires are malnourished and dehydrated, and terrified of humans after hostile encounters with villagers when forced to search further afield for food.

Some have required surgery for infections exacerbated by the haze, Hermansyah said, while others have been too scared to venture for long outside the protection of their enclosures.

- Fires 'beyond crazy' -

The threat level at the centre still sits on "code yellow" but the pace and scale of the approaching fires have forced programme manager Denny Kurniawan to consider an unprecedented scenario -- code red, a full-scale evacuation of all 470 orangutans in their care.

The logistical challenges of such a massive operation are something the carers don't want to dwell on, but Kurniawan fears some in the wild have already burned to death and will not leave others to their fate in the fires.

"This year's disaster is definitely the worst since 1997," Kurniawan said, referring to the worst-recorded haze crisis in history.

"We've never been forced to evacuate orangutans or draw up an emergency contingency plan, but these fires are beyond crazy."

The scale of the disaster has captured world headlines, with the haze now drifting as far as Thai holiday islands and the Philippines. Indonesian President Joko Widodo cut short his trip to the United States on Monday to return home to deal with the crisis.

The government has deployed around 30 aircraft and tens of thousands of troops to fight the fires, and begun sending warships to the worst-affected regions to prepare for large-scale evacuations if needed.

Authorities in Kalimantan have blamed limited resources and tinder-dry conditions they say have made it extremely difficult to control the fires. The head of the provincial nature conservancy agency Nandang Prihadi told AFP that people "must be patient".

But that is not an option for Kurniawan and his staff at the beleaguered centre, who are frustrated that so little progress has been made in nearly two decades of annual haze outbreaks.

"Why haven't we learned anything?" he said. "Why does this keep happening?"