Numerical simulation of combustion phenomena such as flashback involves complex interactions between acoustics, turbulence, and combustion. An approach is proposed which combines a large eddy simulation (LES) technique with an adapted flame representation for premixed combustion. The standard Smagorinsky model is used for the subgrid closure. The flame is artificially thickened so that it can be resolved on the LES mesh. Two-dimensional nonreactive computations are first performed in a backward facing step configuration. Spectral analysis of the pressure and velocity signals recorded during the calculations are compared with experimental results. The mechanism of flashback linked to combustion instabilities in a sudden expansion configuration is then investigated. The effects of the instabilities are simulated by imposing an external velocity modulation at the entrance of the computational domain. Two cases are discussed which illustrate the dynamics of the flame. Upstream propagation is observed in response to strong flow perturbations.