A giant rat the size of a cat has been discovered in the pristine wilderness of western New Guinea's Foja Mountains.

The 1.4kg Mallomys rodent is about five times the size of a common rat, according to the team that found it.

The conservationists, from Conservation International (CI) and Indonesia Institute of Science (LIPI), also discovered a pygmy possum - one of the world's smallest marsupials.

CI and LIPI scientists discovered dozens of new plants and animals on their first expedition to the Indonesian region, described as a "lost world", in late 2005.

The latest expedition took place last June.

"It's comforting to know that there is a place on earth so isolated that it remains the absolute realm of wild nature," said CI vice president Bruce Beehler, who led the expedition.

Kristofer Helgen, Smithsonian Institution: "The giant rat is about five times the size of a typical city rat."

"We were pleased to see that this little piece of Eden remains as pristine and enchanting as it was when we first visited."

Both species are currently being studied to determine whether they are new to science.

"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, DC.

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

During the latest expedition, the team, which were accompanied by a film crew, also recorded the mating displays of several rare and little-known birds for the first time.

The film crew obtained the first film documentation of several spectacular birds found in Foja, capturing on tape the full courtship displays of the golden-fronted bowerbird (Amblyornis flavifrons) and of the black sicklebill bird of paradise (Epimachus fastuosus).

They also recorded the "lost" Bird of Paradise - Parotia berlepschi (known as Berlepsch's six-wired bird of paradise), and the newly described wattled smoky honeyeater (Melipotes carolae), both known only from the Foja Mountains.

The Foja Wilderness is part of the great Mamberamo Basin, the largest unroaded tropical forest in the Asia Pacific region.

CI and LIPI said they have already planed another expedition back to the Foja Mountains in late 2008 or 2009.

They expect to find additional new species of frogs, mammals, butterflies and plants.