Fires that have ravaged some 20,000 hectares of forest in Argentina's remote Patagonia region were deliberately set, according to Argentine officials on Monday.

"It is a clear case of arson. The fires were set," said Anibal Fernandez, President Cristina Kirchner's cabinet chief, who said authorities suspect real estate speculation as the motive.

More than 400 firefighters worked over the weekend to contain some of the fires at the southern end of Argentina, a rugged and sparsely populated region that it popular with tourist and outdoorsmen.

The areas partly destroyed include a large tract of land in Argentina's Lago Puelo National Park, one of the nation's most prized nature preserves.

Officials said crews are still battling five other fires that have proved more difficult to contain.

The first of the blazes were set about two weeks ago, authorities said.

Fernandez hinted that there may have been a financial motive for setting the fire, noting that the land was "not worth much as long as it is covered with trees."

Once cleared, the land can be exploited for development, he said.

"Once those tree are no longer there, the land can be sold," the official added.