  We show that a standard model gauge singlet fermion field, with mass of order
keV or larger, and involved in the inverse seesaw mechanism of light neutrino
mass generation, can be a good warm dark matter candidate. Our framework is
based on B-L extension of the Standard Model. The construction ensures the
absence of any mixing between active neutrinos and the aforementioned dark
matter field. This circumvents the usual constraints on the mass of warm dark
matter imposed by X-ray observations. We show that over-abundance of thermally
produced warm dark matter (which nevertheless do not reach chemical
equilibrium) can be reduced to an acceptable range in the presence of a moduli
field decaying into radiation --- though only when the reheat temperature is
low enough. Our warm dark matter candidate can also be produced directly from
the decay of the moduli field during reheating. In this case, obtaining the
right amount of relic abundance, while keeping the reheat temperature high
enough as to be consistent with Big Bang nucleosynthesis bounds, places
constraints on the branching ratio for the decay of the moduli field into dark
matter.
