Context: The existence in the Milky Way of either a long thin bar with a half length of 4.5 kpc and a position angle of around 45° with respect to the Sun-Galactic centre line or a bulge+bar complex, thicker and shorter, with a smaller tilt respect to the Sun-GC line, has been a matter of discussion in recent decades. Aims: In this paper, we present low resolution (R=500) near-infrared spectra for selected and serendipitous sources in six inner in-plane Galactic fields at l=7°, 12°, 15°, 20°, 26° and 27°, with the aim of analysing the stellar content present in those fields. Methods: From the equivalent widths of the main features of the K band spectra (the NaI, CaI and CO bandheads) we have derived the metallicities of the sources by means of the empirical scale obtained by Ramírez et al. (2000, AJ, 120, 833) and Frogel et al. (2001, AJ, 122, 1896) for luminous red giants. Results: Our results show how the mean metallicity of the sample varies with Galactic longitude. We find two groups of stars, one whose [Fe/H] is similar to the values obtained for the bulge in other studies (Molla et al. 2000, MNRAS, 316, 345; Schultheis et al. 2003, A&A, 405, 531), and a second one with a metallicity similar to that of the inner parts of the disc (Rocha-Pinto et al. 2006, A&A, 453, L9). The relative density of both groups of stars in our sample varies in a continuous way from the bulge to the disc. This could hint at the existence of a single component apart from the disc and bulge, running from l=7° to l=27° and able to transport disc stars inwards and bulge stars outwards, which could be the Galactic bar that has been detected in previous works as an overdensity of stars located at those galactic coordinates (Hammersley et al. 1994, MNRAS, 269, 753; Hammersley et al. 2000, MNRAS, 317, L45; Picaud et al. 2003, A&A, 408, 141). Appendices A and B are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/479/131