  We consider the properties of the gauge-invariant two-point correlation
functions of the gauge-field strengths for QCD in the presence of a magnetic
background field. We discuss the general structure of the correlators in this
case and provide the results of an exploratory lattice study for $N_f = 2$ QCD
discretized with unimproved staggered fermions. Our analysis provides evidence
for the emergence of anisotropies in the non-perturbative part of the
correlators and for an increase of the gluon condensate as a function of the
external magnetic field.
