  The frequencies of transverse-optical $\Gamma$ phonons in KNbO$_3$ and
KTaO$_3$ are calculated in the frozen-phonon scheme making use of the
full-potential linearized muffin-tin orbital method. The calculated frequencies
in the cubic phase of KNbO$_3$ and in the tetragonal ferroelectric phase are in
good agreement with experimental data. For KTaO$_3$, the effect of lattice
volume was found to be substantial on the frequency of the soft mode, but
rather small on the relative displacement patterns of atoms in all three modes
of the $T_{1u}$ symmetry. The TO frequencies in KTaO$_3$ are found to be of the
order of, but somehow higher than, the corresponding frequencies in cubic
KNbO$_3$.
