Ever since their discovery , Infrared dark clouds ( IRDCs ) are generally considered to be the sites just at the onset of high-mass ( HM ) star formation .
In recent years , it has been realized that not all IRDCs harbour HM Young Stellar Objects ( YSOs ) .
Only those IRDCs satisfying a certain mass-size criterion , or equivalently above a certain threshold density , are found to contain HMYSOs .
In all cases , IRDCs provide ideal conditions for the formation of stellar clusters .
In this paper , we study the massive stellar content of IRDCs to re-address the relation between IRDCs and HM star formation .
For this purpose , we have identified all IRDCs associated to a sample of 12 Galactic molecular clouds ( MCs ) .
The selected MCs have been the target of a systematic search for YSOs in an earlier study .
The catalogued positions of YSOs have been used to search all YSOs embedded in each identified IRDC .
In total , we have found 834 YSOs in 128 IRDCs .
The sample of IRDCs have mean surface densities of 319 M _ { \odot } ~ { } { pc } ^ { -2 } , mean mass of 1062 M _ { \odot } , and a mass function power-law slope -1.8 , which are similar to the corresponding properties for the full sample of IRDCs and resulting physical properties in previous studies .
We find that all those IRDCs containing at least one intermediate to high-mass young star satisfy the often-used mass-size criterion for forming HM stars .
However , not all IRDCs satisfying the mass-size criterion contain HM stars .
We find that the often used mass-size criterion corresponds to 35 % probability of an IRDC forming a massive star .
Twenty five ( 20 % ) of the IRDCs are potential sites of stellar clusters of mass more than 100 M _ { \odot } .