  We determine theoretically the interaction between two magnetic impurities
embedded in a spin-split $s$-wave superconductor. The spin-splitting in the
superconductor gives rise to two different interaction types between the
impurity spins, depending on whether their spins lie in the plane perpendicular
to the spin-splitting field (Heisenberg) or not (Ising). For impurity
separation distances exceeding $\xi_S$, we find that the magnitude of the
spin-splitting can determine whether an antiferromagnetic or ferromagnetic
alignment of the impurity spins is preferred by the RKKY interaction. Moreover,
the Ising and Heisenberg terms of the RKKY interaction alternate on being the
dominant term and their magnitudes oscillate as a function of distance between
the impurities.
