Fires in Indonesia, largely blamed on slash-and-burn farming practices, have created unhealthy smog across Southeast Asia. The country has spent months this year battling blazes as the El Nino weather pattern has extended the annual dry season.

"Many of the palm oil and pulp groups with the largest burned areas in their concessions have either not received any serious civil/administrative sanctions, or have had sanctions imposed that do not appear to fit with the level or frequency of burning," the environmental action group said in its report.

Rasio Ridho Sani, director general of law enforcement at Indonesia's Environment Ministry, said law enforcement had been "very strict" through administrative sanctions, including revoking licences and civil lawsuits.

He said on Monday that Indonesia was studying a plan for imposing harsher penalties on firms found burning forest and peat land.

The government sealed off burned areas in concessions controlled by 52 companies and the authorities were investigating five companies on suspicion of starting fires or being negligent in containing fires on their land, he said.

Greenpeace said 3.4 million hectares had been burned from 2015 to 2018 in Indonesia, with 2.6 million hectares burned in 2015 alone.

Citing analysis of mapping and concession data, Greenpeace said concessions linked to three pulpwood companies saw about 403,300 hectares - an area more than five times the size of Singapore - burned from 2015 to 2018.

Concessions linked to the companies include subsidiaries, units, suppliers or affiliates, Greenpeace said.

Greenpeace said Sinar Mas and its subsidiary Asia Pulp & Paper accounted for nearly two-thirds of the 403,000 hectares and said the other two firms were APRIL Group and state-owned Perhutani.

APRIL said its evaluation of the data showed the burnt area in its concessions was "a fraction of Greenpeace's estimate".

Asia Pulp & Paper said the numbers published by Greenpeace were inaccurate as some of the companies listed as its suppliers were not its suppliers.

Greenpeace said the three groups were handed a total of 24 civil or administrative sanctions, but said this had not always covered the largest or most serious burnt areas. It cited government data it gained via freedom of information requests.

The group said none of the 10 palm oil concessions with the largest areas of burned land in 2015-2018 received any serious sanctions. Seven of those concessions had high numbers of hotspots, which show up on mapping images and indicate a fire, this year, Greenpeace said.