  The nature of the low- to intermediate-luminosity (Lx$\,\sim 10^{32-34}$ erg
s$^{-1}$) source population revealed in hard band (2-10 keV) X-ray surveys of
the Galactic Plane is poorly understood. To overcome such problem we
cross-correlated the XMM-Newton 3XMM-DR4 survey with the infrared 2MASS and
GLIMPSE catalogues. We identified reliable X-ray-infrared associations for 690
sources. We selected 173 sources having hard X-ray spectra, typical of hard
X-ray high-mass stars (kT$\,>\,5\,$keV), and 517 sources having soft X-ray
spectra, typical of active coronae. About $18\,\%$ of the soft sources are
classified in the literature: $\sim\,91\%$ as stars, with a minor fraction of
WR stars. Roughly $15\,\%$ of the hard sources are classified in the
literature: $\sim\,68\%$ as high-mass X-ray stars single or in binary systems
(WR, Be and HMXBs), with a small fraction of G and B stars. We carried out
infrared spectroscopic pilot observations at the William Herschel Telescope for
five hard X-ray sources. Three of them are high-mass stars with spectral types
WN7-8h, Ofpe/WN9 and Be, and Lx$\sim\,10^{32}-10^{33}$erg s$^{-1}$. One source
is a colliding-wind binary, while another source is a colliding-wind binary or
a supergiant fast X-ray transient in quiescence. The Be star is a likely
$\gamma$-Cas system. The nature of the other two X-ray sources is uncertain.
The distribution of hard X-ray sources in the parameter space made of X-ray
hardness ratio, infrared colours and X-ray-to-infrared flux ratio suggests that
many of the unidentified sources are new $\gamma$-Cas analogues, WRs and low Lx
HMXBs. However, the nature of the X-ray population with Ks $\geq$ 11 and
average X-ray-to-infrared flux ratio remains unconstrained.
