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Two species of mammals that may be new to science have been discovered in a remote region of Indonesia which has been labelled a "lost world" because much of much of it remains virtually unexplored and uninhabited.

The pygmy possum and giant rat were found during a scientific expedition earlier this year to the Foja Mountains of western New Guinea - a pristine wilderness which has already revealed a rich array of novel flora and fauna.

The tree-dwelling pygmy possum belongs to the Cercartetus group of marsupials and is one of the world's smallest.

The giant rat is about the size of a cat and has been classified as belonging to the Mallomys genus of woolly rodents.

"The giant rat is about five times the size of a typical city rat," said Kristofer Helgen, a scientist with the Smithsonian Institution in Washington.

"With no fear of humans, it apparently came into the camp several times during the trip."

The Foja wilderness is part of the great Mamberamo Basin and is one of the least disturbed regions in the Asia-Pacific region.

It has the largest area of tropical forest without roads or trails.

Its isolation is helped by its steep mountainous slopes and difficult terrain.

Bruce Beehler, the vice-president of the Conservation International group, which is based in Washington, said that the latest expedition had continued to make the sort of astonishing discoveries that were documented in earlier field trips to the region, which covers an area of about 300,000 hectares.

"It is comforting to know that there is a place on Earth so isolated it remains the absolute realm of wild nature," he added.

"We were pleased to see this little piece of Eden remains as pristine and enchanting as it was when we first visited." Scientists are studying the pygmy possum and the giant rat in more detail to ascertain whether they are genuinely new species and, if so, where they fit in with their closest relatives.

It is rare for scientists to find new species of large mammals, although a dozen or so have been found over the past couple of decades in south-east Asia.

The Foja Mountains rise to about 2,200 metres and are largely uninhabited.

Tribes live nearer sea level and abundant game near their villages means they do not often venture further afield up the steep slopes.

Western scientists have made several trips to the region in recent years with the help of helicopters.

The first explorers described being surrounded by giant flowers and unknown plants with tree kangaroos and spiny anteaters who appeared to be unafraid of humans - indicating no previous contact.

"It is as close to the Garden of Eden as you're going to find on Earth," Dr Beehler said after the first expedition in 2006.

"We found dozens, if not hundreds, of new species in what is probably the most pristine ecosystem in the whole Asia-Pacific region. There were so many new things it was almost overwhelming. And we have only scratched the surface of what it is there."

In the latest trip, reported in National Geographic , the team filmed spectacular birds for the first time, such as the courtship display of the golden-fronted bowerbird and the black sicklebill bird of paradise.