[sfepy] Set groups in vtk mesh.

Hello. Is there a way to somehow mark some vertices in vtk mesh so I could later use them in region definition? I've tried to create my vtk mesh with vertices field "node_groups". And some vertices have this field to set to 1, and other to 2. But it doesnt work. Sfepy tell me

raise ValueError('region "%s" has no entities!' % self.name) Thank you