  We investigate vacuum expectation value of the energy-momentum tensor for a
massive Dirac field in flat spacetime with a toroidal subspace of a general
dimension. Quasiperiodicity conditions with arbitrary phases are imposed on the
field operator along compact dimensions. These phases are interpreted in terms
of magnetic fluxes enclosed by compact dimensions. The equation of state in the
uncompact subspace is of the cosmological constant type. It is shown that, in
addition to the diagonal components, the vacuum energy-momentum tensor has
nonzero off-diagonal components. In special cases of twisted (antiperiodic) and
untwisted (periodic) fields the off diagonal components vanish. For untwisted
fields the vacuum energy density is positive and the energy-momentum tensor
obeys the strong energy condition. For general values of the phases in the
periodicity conditions the energy density and stresses can be either positive
or negative. The numerical results are given for a Kaluza-Klein type model with
two extra dimensions.
