  Studying the physical conditions in circumstellar disks is a crucial step
toward understanding planet formation. Of particular interest is the case of HD
100546, a Herbig Be star that presents a gap within the first 13 AU of its
protoplanetary disk, that may originate in the dynamical interactions of a
forming planet. We gathered a large amount of new interferometric data using
the AMBER/VLTI instrument in the H- and K-bands to spatially resolve the warm
inner disk and constrain its structure. Then, combining these measurements with
photometric observations, we analyze the circumstellar environment of HD 100546
in the light of a passive disk model based on 3D Monte-Carlo radiative
transfer. Finally, we use hydrodynamical simulations of gap formation by
planets to predict the radial surface density profile of the disk and test the
hypothesis of ongoing planet formation. The SED and the NIR interferometric
data are adequately reproduced by our model. We show that the H- and K-band
emissions are coming mostly from the inner edge of the internal dust disk,
located near 0.24 AU from the star, i.e., at the dust sublimation radius in our
model. We directly measure an inclination of $33^{\circ} \pm 11^{\circ}$ and a
position angle of $140^{\circ} \pm 16^{\circ}$ for the inner disk. This is
similar to the values found for the outer disk ($i \simeq 42^{\circ}$, $PA
\simeq 145^{\circ}$), suggesting that both disks may be coplanar. We finally
show that 1 to 8 Jupiter mass planets located at $\sim 8$ AU from the star
would have enough time to create the gap and the required surface density jump
of three orders of magnitude between the inner and outer disk. However, no
information on the amount of matter left in the gap is available, which
precludes us from setting precise limits on the planet mass, for now.
