  Spectroscopic multiplicity surveys of O stars in young clusters and OB
associations have revealed that a large portion ($\sim$ 70%) of these massive
stars (M$_{i}$ $\gt$ 15 $M_{\odot}$) belong to close and short-period binaries
(physical separation d $\lt$few au). Despite the recent and significant
progress, the formation mechanisms leading to such close massive multiple
systems remain to be elucidated. As a result, young massive close binaries (or
higher-order multiple systems) are unique laboratories to figure out the
pairing mechanism of high-mass stars. We present the first VLTI/GRAVITY
observations of six young O stars in the M17 star-forming region ($\lesssim$ 1
Myr) and two additional foreground stars. From the interferometric model
fitting of visibility amplitudes and closure phases, we search for companions
and measure their positions and flux ratios. Combining the resulting magnitude
difference with atmosphere models and evolutionary tracks, we further constrain
the masses of the individual components. All of the six high-mass stars are in
multiple systems, leading to a multiplicity fraction (MF) of 100%, yielding a
68% confidence interval of 94-100%. We detect a total number of 9 companions
with separations up to 120 au. Including previously identified spectroscopic
companions, the companion fraction of the young O stars in our sample reaches
2.3$\pm$0.6. The derived masses span a wide range from 2.5 to 50 $M_{\odot}$,
with a great tendency towards high-mass companions. While based on a modest
sample, our results clearly indicate that the origin of the high degree of
multiplicity is rooted in their star formation mechanism. No clear evidence for
one of the competing concepts of massive star formation (core accretion or
competitive accretion) could be found. However, our results are compatible with
migration as a scenario for the formation of close massive binaries.
