QGIS calculate the 3d surface area of a region

I would like to calculate the true surface area of a region from DEM (2d elevation data). Is it possible to do this with QGIS? And if yes, how? I managed to obtain a 3d vizualization of the elevation data using nviz (a GRASS-module) but i do not see any possibility to caluclate the 3D surface area.

Thank you very much in advance! Olga