  We present medium resolution ($R\approx5300$) $K$-band integral field
spectroscopy of six MYSOs. The targets are selected from the RMS survey, and we
used the NIFS on the Gemini North telescope. The data show various spectral
line features including Br$\gamma$, CO, H$_2$, and \mbox{He\,{\sc i}}. The
Br$\gamma$ line is detected in emission in all objects with
$v_\mathrm{FWHM}\sim100$ -- 200 kms$^{-1}$. V645 Cyg shows a high-velocity
P-Cygni profile between -800 kms$^{-1}$ and -300 kms$^{-1}$. We performed
three-dimensional spectroastrometry to diagnose the circumstellar environment
in the vicinity of the central stars using the Br$\gamma$ line. We measured the
centroids of the velocity components with sub-mas precision. The centroids
allow us to discriminate the blueshifted and redshifted components in a roughly
east--west direction in both IRAS 18151--1208 and S106 in Br$\gamma$. This lies
almost perpendicular to observed larger scale outflows. We conclude, given the
widths of the lines and the orientation of the spectroastrometric signature,
that our results trace a disc wind in both IRAS 18151--1208 and S106. The CO
$\nu=2-0$ absorption lines at low $J$ transitions are detected in IRAS
18151--1208 and AFGL 2136. We analysed the velocity structure of the neutral
gas discs. In IRAS 18151--1208, the absorption centroids of the blueshifted and
redshifted components are separated in a direction of north-east to south-west,
nearly perpendicular to that of the larger scale $H_2$ jet. The
position-velocity relations of these objects can be reproduced with central
masses of 30 $M_{\sun}$ for IRAS 18151--1208 and 20 $M_{\sun}$ for AFGL 2136.
We also detect CO $\nu=2-0$ bandhead emission in IRAS 18151--1208, S106 and
V645 Cyg. The results can be fitted reasonably with a Keplerian rotation model,
with masses of 15, 20 and 20 $M_{\sun}$ respectively.
