  [Abridged] Twenty-six high-luminosity IRAS sources believed to be collection
of stars in the early phases of high-mass star formation have been observed in
the NIR (J, H, K) to characterize the clustering properties of their young
stellar population and gain insight into the initial conditions of star
formation in these clusters (Initial Mass Function [IMF], Star Formation
History [SFH]), and to deduce mean values for cluster ages.
  K luminosity functions (KLFs) are compared with simulated ones from a model
that generates populations of synthetic clusters starting from assumptions on
the IMF, the SFH, and the Pre-MS evolution, and using the average properties of
the observed clusters as boundary conditions
  Twenty-two sources show evidence of clustering from a few up to several tens
of objects, and a median cluster radius of 0.7 pc. A considerable number of
cluster members present an infrared excess characteristic of young
Pre-Main-Sequence objects. We find that the median stellar age ranges between
2.5 10^5 and 5 10^6 years, with evidence of an age spread of the same entity
within each cluster. We also find evidence that older clusters tend to be
smaller in size, in line with the fact that our clusters are on average larger
than those around relatively older Herbig Ae/Be stars. The relationship of the
mass of the most massive star in the cluster with both the clusters richness
and their total stellar mass suggest that our modeled clusters may not be
consistent with them resulting from random sampling of the IMF.
  Our results are consistent with a star formation which takes place
continuously over a period of time which is longer than a typical crossing
time.
