Context : 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 Near-IR ( J , H , K _ { \mathrm { s } } ) to characterize the clustering properties of their young stellar population and compare them with those of more evolved objects ( e.g. , Herbig Ae/Be stars ) of comparable mass .
All the observed sources possess strong continuum and/or line emission in the millimeter , being therefore associated with gas and dust envelopes .
Nine sources have far-IR colors characteristic of UCHII regions while the other 17 are likely being experiencing an evolutionary phase that precedes the Hot-Cores , as suggested by a variety of evidence collected in the past decade .

Aims : To 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 .

Methods : For each cluster we carry out aperture photometry .
We derive stellar density profiles , color-color and color-magnitude diagrams , and color ( HKCF ) and luminosity ( KLF ) functions .
These two functions are compared with simulated KLFs and HKCFs 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 ( bolometric luminosity , dust distribution , infrared excess , extinction ) .

Results : Twenty-two sources show evidence of clustering with a stellar richness indicator that varies 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 .
For a subset of 9 detected clusters , we could perform a statistically significant comparison of the observed KLFs with those resulting from synthetic cluster models ; for these clusters we find that the median stellar age ranges between 2.5 \cdot 10 ^ { 5 } and 5 \cdot 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 .
Our models allow us to explore the relationship of the mass of the most massive star in the cluster with both the clusters richness and their total stellar mass .
Although such relationships are predicted by several classes of cluster formation models , their detailed analysis suggests that our modeled clusters may not be consistent with them resulting from random sampling of the IMF .

Conclusions : 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 .