Context : The inner regions of the discs of high-mass young stellar objects ( HMYSOs ) are still poorly known due to the small angular scales and the high visual extinction involved .

Aims : We deploy near-infrared ( NIR ) spectro-interferometry to probe the inner gaseous disc in HMYSOs and investigate the origin and physical characteristics of the CO bandhead emission ( 2.3–2.4 \mu m ) .

Methods : We present the first GRAVITY/VLTI observations at high spectral ( \mathcal { R } =4000 ) and spatial ( mas ) resolution of the CO overtone transitions in NGC 2024 IRS 2 .

Results : The continuum emission is resolved in all baselines and is slightly asymmetric , displaying small closure phases ( \leq 8 \degr ) .
Our best ellipsoid model provides a disc inclination of 34 \degr \pm 1 \degr , a disc major axis position angle ( PA ) of 166 \degr \pm 1 \degr , and a disc diameter of 3.99 \pm 0.09 mas ( or 1.69 \pm 0.04 au , at a distance of 423 pc ) .
The small closure phase signals in the continuum are modelled with a skewed rim , originating from a pure inclination effect .
For the first time , our observations spatially and spectrally resolve the first four CO bandheads .
Changes in visibility , as well as differential and closure phases across the bandheads are detected .
Both the size and geometry of the CO-emitting region are determined by fitting a bidimensional Gaussian to the continuum-compensated CO bandhead visibilities .
The CO-emitting region has a diameter of 2.74 \pm ^ { 0.08 } _ { 0.07 } mas ( 1.16 \pm 0.03 au ) , and is located in the inner gaseous disc , well within the dusty rim , with inclination and PA matching the dusty disc geometry , which indicates that both dusty and gaseous discs are coplanar .
Physical and dynamical gas conditions are inferred by modelling the CO spectrum .
Finally , we derive a direct measurement of the stellar mass of M _ { * } \sim 14.7 ^ { +2 } _ { -3.6 } M _ { \sun } by combining our interferometric and spectral modelling results .

Conclusions :