On a question of O'Grady about modified diagonals

Let X be an abelian variety of dimension g. In a recent preprint O'Grady defines modified diagonal classes \Gamma^m on X^m and he conjectures that the class of \Gamma^m in the Chow ring of X^m is torsion for m \geq 2g+1. We prove a generalization of this conjecture.