Illegal slash and burn clearance for palm oil plantations is threatening orangutans in one of Sumatra's richest habitats, conservationists say.

An area of swampland containing the highest density of the red apes on the planet is being illegally cleared for the production of palm oil, used in countless packaged food products found on the world's supermarket shelves, they said.

"If we can't stop them here, then there really is no hope," Ian Singleton, who heads the Sumatra Orangutan Conservation Program, told NBC News.

A 2008 survey by Singleton and other scientists found fewer than 7,000 of the critically endangered Sumatran orangutans left in the wild, most of them living in a large region of swampland and lowland forest at the northern tip of the Indonesian island of Sumatra.

Much of the area has already been converted to palm oil plantations.

Plantation owners often regard orangutans as pests, Singleton said, although some profit by illegally selling off babies as pets.

"The law is very clear, but the enforcement is very weak," Singleton said.

Using fire in a slash and burn process to clear the land in what is a protected forest is illegal under Indonesian law but the country has a poor track record in enforcing its own environmental laws, conservationists said.