  Context: A rich population of low-mass brown dwarfs and isolated planetary
mass objects has been reported recently in the Upper Scorpius and Ophiuchus
star forming complex. Aims: We investigate the membership, nature and
properties of 17 of these isolated planetary mass candidates using
low-resolution near-infrared spectra. Methods: We investigate the membership by
looking for evidences of youth using four diagnostics: the slope of the
continuum between the J and Ks band, the Hcont and TLI-g gravity sensitive
indices, and by comparing the spectra to young and field (old) M and L-dwarf
standards. Results: All the targets but one are confirmed as young ultracool
objects, with spectral types between L0 and L6 and masses in the range
0.004-0.013 M according to evolutionary models. The status of the last target
is unclear at this point. Conclusions: Only one possible contaminant has been
identified among the 17 targets, suggesting that the contamination level of the
original sample must be lower than 6%
