  We consider a periodic self-adjoint pseudo-differential operator
$H=(-\Delta)^m+B$, $m>0$, in $\R^d$ which satisfies the following conditions:
(i) the symbol of $B$ is smooth in $\bx$, and (ii) the perturbation $B$ has
order less than $2m$. Under these assumptions, we prove that the spectrum of
$H$ contains a half-line. This, in particular implies the Bethe-Sommerfeld
Conjecture for the Schr\"odinger operator with a periodic magnetic potential in
all dimensions.
