At the onset of high-mass star formation, accreting protostars are deeply embedded in massive cores made of gas and dust. Their spectral energy distribution is still dominated by the cold dust and rises steeply from near-to far-infrared wavelengths. The young massive star-forming region IRDC18223-3 is a prototypical Infrared-Dark-Cloud with a compact mm continuum core that shows no protostellar emission below 8mum. However, based on outflow tracers, early star formation activity was previously inferred for this region. Here, we present recent Spitzer observations from the MIPSGAL survey that identify the central protostellar object for the first time at 24 and 70mum. Combining the mid- to far-infrared data with previous mm continuum observations and the upper limits below 8mum, one can infer physical properties of the central source. At least two components with constant gas mass M and dust temperature T are necessary: one cold component (~15K and ~576M_sun) that contains most of the mass and luminosity, and one warmer component (>=51K and >=0.01M_sun) to explain the 24mum data. The integrated luminosity of ~177L_sun can be used to constrain additional parameters of the embedded protostar from the turbulent core accretion model for massive star formation. The data of IRDC18223-3 are consistent with a massive gas core harboring a low-mass protostellar seed of still less than half a solar mass with high accretion rates of the order 10^-4M_sun/yr. In the framework of this model, the embedded protostar is destined to become a massive star at the end of its formation processes.

5 pages, 2 figures, accepted for Astrophysical Journal Letters, for a high-resolution version see http://www.mpia.de/homes/beuther/papers.html