  The RKKY interaction between rare-earth (RE) ions in high-$T_c$
superconductors is considered at $T\ll T_c$. It is shown that this interaction
consists of two terms: conventional oscillating one and the positive term,
which is proportional to the gap function and decreases in the $2D$ case
inversely proportional to the distance. In the antiferromagnetic state of the
RE subsystem this positive interaction gives rise for frustrations which
diminishes the Neel temperature. In the case of strongly anisotropic gap
function this frustration produces two different values of the effective
nearest neighbor exchange coupling between RE ions along the $a$ and $b$. This
anisotropy has been established experimentally in Ref.\cite{6,7,8}.
