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We present low-resolution (R=900) optical (576.1--1,051.1 nm) spectroscopicobservations of 40 candidate very low-mass members in the Upper Scorpius OBassociation. These objects were selected using the $I$, $J$ and $K$ photometryavailable in the DENIS database. We have derived spectral types and we havemeasured H$\alpha$ and NaI doublet (at 818.3 and 819.5 nm) equivalent widths.We assess the youth of the objects by comparing them to their oldercounterparts of similar spectral type in the Pleiades cluster and the field.Our analysis indicates that 28 of our targets are young very low-mass objects,and thus they are strong candidate members of the OB association. The other 12DENIS sources are foreground M dwarfs or background red giants. Our sample ofspectroscopic candidate members includes 18 objects with spectral types in therange M6.5 and M9, which are likely young brown dwarfs. We classify thesecandidates as accreting/non accreting using the scheme proposed by Barrado yNavascu\'es & Mart\'\i n (2003). We find 5 substellar-mass candidate clustermembers that are still undergoing mass accretion, indicating that the timescalefor accretion onto brown dwarfs can be as long as 5 Myr in some cases.Comment: Accepted for publication in The Astronomical Journal, January 200

Spectroscopic Identification of DENIS-selected Brown Dwarf Candidates in the Upper Scorpius OB Association