  We consider how a vertex operator algebra can be extended to an abelian
intertwining algebra by a family of weak twisted modules which are {\em simple
currents} associated with semisimple weight one primary vectors. In the case
that the extension is again a vertex operator algebra, the rationality of the
extended algebra is discussed. These results are applied to affine Kac-Moody
algebras in order to construct all the simple currents explicitly (except for
$E_8$) and to get various extensions of the vertex operator algebras associated
with integrable representations.
