Fires raging in an Indonesian swamp forest may have killed a third of the rare Sumatran orangutans living there and the rest could die this year, conservationists warned on Wednesday.

The Tripa forest in Aceh province is home to the world's densest population of the critically endangered species. At last count, about 200 lived there, out of a world population of 6,600, the conservationists said.

Cloud-free images from December showed that just over 12,000 hectares (30,000 acres) of the original 60,000 hectares of forest remains, said Graham Usher of the Foundation for a Sustainable Ecosystem.

The rest has been broken up and degraded as palm oil companies drain the swamp, he said. He said a total of 92 fire hot spots were recorded between 19 and 25 March in several of the palm oil plantations in the area.

"If there is a prolonged drought and the fire continues … then orangutans, tigers and sun bears within it will be exterminated before the end of 2012," he told a news conference, held by the Coalition to Save Tripa, which includes Greenpeace.

The number of orangutans killed in recent months could be 100 or more, the groups estimated. Another 100 died between 2009 and 2011 – killed either in the conversion process or by starvation and malnutrition.

Ian Singleton, conservation director of the Sumatran Orangutan Conservation Programme, said there are fewer than 200 orangutans still in Tripa, which was home to 3,000 individuals in early 1990.

"It is no longer several years away, but just a few months or even weeks before this iconic creature disappears," he said. "We are currently watching a global tragedy."

He described those kept illegally as pets as the "lucky" ones but said they will be refugees from a forest that no longer exists.

Last year, Aceh's governor, Irwandi Yusuf, gave a licence to PT Kallista Alam, a palm oil company, to convert 1,600 hectares of the Tripa peat swamp, home of orangutans, tigers and bears. Three other companies are already operating in the area.

An environmental group has filed both a criminal complaint and a lawsuit against the government. The Aceh administrative court is expected to rule on this next week.