A woman wears a face mask as haze shrouded South Sumatra, Indonesia. The air pollution is caused by the illegal burning of forest and peat fires to clear new land for the production of pulp, paper and palm oil.

A conservation scientist has described an acrid haze choking parts of South-east Asia as one of the worst ever man-made environmental disasters, saying that it is affecting about 40 million people.

Eric Meijaard, an associate professor at the University of Queensland, warns the pollution levels of toxic carbon monoxide and ground-level ozone are "presently off the scale".

"Not only is there appalling human suffering, with hundreds of thousands of people ill and many dead, the fires are a massive economic cost to the Indonesian economy," said Professor Meijaard, an expert on Indonesia's forests who co-ordinates the environmental program Borneo Futures.

ULET IFANSASTI/GETTY IMAGES A man tries to extinguish a fire on burned peatland and fields near his house in South Sumatra. The haze has been an annual problem for the past 18 years in Indonesia.

Thousands of fires caused by slash-and-burn farming have blanketed parts of Indonesia, Singapore and Malaysia for almost two months, forcing many schools to close and flights and international events to be cancelled.

The haze has spread to southern Thailand, with record high pollution levels.

"This is a crisis," said Halam Jemarican, head of Thailand's Southern Environment Office.

The haze has also choked Brunei and even reached parts of Cambodia, the Philippines and Vietnam.

According to the World Resources Institute the fires, mainly in south Sumatra and south and central Kalimantan, have caused carbon emissions that have surpassed those of the entire United States - the world's second largest emitter of greenhouse gases - on 26 out of 44 days since September.

Meijaard said he believes Indonesia's government has failed to recognise the seriousness of the problem.

"Are the few helicopters and water bombers, an insufficient supply of the right type of face masks, and some canals dug in the peat to guide waters to fires really the best the government can do?"

Meijaard asked why Indonesia has not sent in a million soldiers to address the disaster. "Why is all government action so apparently lacklustre and unfocused?"

The professor said what might help garner action is to call the disaster what it really is: the biggest environmental crime of the 21st century.

"BP's Deepwater Horizon oil spill of 2010 looks relatively benign compared to Indonesia's 2015 fire crisis … and that was one heck of a spill," he said.

"And I consider it a crime, not just a disaster, because even though setting fire to land remains perfectly legal in Indonesia, endangering the lives of millions of people, destroying protected forests and their wildlife, and threatening the global environment are criminal acts."

Meijaard called for efforts including fire bans, improved firefighting efforts, a prohibition on peat development and programs to divest agricultural production on peat.

"If the government of Indonesia cares for its people, its economy, its wildlife and for people elsewhere in the world, it immediately must do more," he said.