Intermediate-mass black holes (IMBHs) in the mass range bridge the gap between stellar black holes (BHs) and supermassive BHs. Here, we investigate the possibility that IMBHs form in young star clusters via runaway collisions and BH mergers. We analyse 10(4) simulations of dense young star clusters, featuring up-to-date stellar wind models and prescriptions for core collapse and (pulsational) pair instability. In our simulations, only nine IMBHs out of 218 form via binary BH mergers, with a mass similar to 100-140 M-circle dot. This channel is strongly suppressed by the low escape velocity of our star clusters. In contrast, IMBHs with masses up to similar to 438 M-circle dot efficiently form via runaway stellar collisions, especially at low metallicity. Up to similar to 0.2 percent of all the simulated BHs are IMBHs, depending on progenitor's metallicity. The runaway formation channel is strongly suppressed in metal-rich (Z = 0.02) star clusters, because of stellar winds. IMBHs are extremely efficient in pairing with other BHs: similar to 70 percent of them are members of a binary BH at the end of the simulations. However, we do not find any IMBH-BH merger. More massive star clusters are more efficient in forming IMBHs: similar to 8 percent (similar to 1 percent) of the simulated clusters with initial mass 10(4)-3 x 10(4) M-circle dot (10(3)-5 x 10(3) M-circle dot) host at least one IMBH.
