We report ultrafast pump-probe spectroscopy examining exciton dynamics in atomically thin MoS${}_{2}$. Spectrally and temporally resolved measurements are performed to investigate the interaction dynamics of two important direct-gap excitons ($A$ and $B$) and their associated broadening kinetics. The two excitons show strongly correlated interexcitonic dynamic, in which the transient blue-shifted excitonic absorption originates from the internal A-B excitonic interaction. The observed complex spectral response is determined by the exciton collision-induced linewidth broadening; the broadening of the $B$-exciton linewidth in turn lowers the peak spectral amplitude of the $A$ exciton. Resonant excitation at the $B$-exciton energy reveals that interexcitonic scattering plays a more important role in determining the broadening kinetics than free-carrier scattering.