A 14-year-old French boy has admitted to setting a massive forest fire that continued to blaze in southern France on Monday, prosecutors said.

The boy was detained on Sunday after the fire broke out close to the village of Orgon, near the city of Avignon, and was quickly spread by the mistral, the strong wind that frequently barrels down the Rhone valley.

He confessed to setting the fire and his detention was extended until Tuesday, a source in the prosecutors' office said.

A source close to the investigation said the boy had previously been charged with setting fires in the area in July.

Local officials said the blaze was under control but still active, with 700 firefighters working to battle it.

By Monday the fire had burnt around 400 hectares (900 acres) of vegetation in the popular tourist region and destroyed two houses but had not caused any casualties, officials said.