  String equations for the vertex operators of type IIB on $AdS_3\times S^3$
background with NS-NS flux are calculated using Berkovits-Vafa-Witten
formalism. With suitable field definitions, the linearized field equations for
six-dimensional supergravity and a tensor multiplet on $AdS_3\times S^3$ are
recovered from these. We also discuss the massless degrees of freedom that
survive the $S^3$ Kaluza-Klein compactification and how our vertex operators
are related to the vertex operators introduced by Giveon, Kutasov and Seiberg.
