  We develop efficient group-theoretical approach to the problem of
classification of evolution equations that admit non-local transformation
groups (quasi-local symmetries), i.e., groups involving integrals of the
dependent variable. We classify realizations of two- and three-dimensional Lie
algebras leading to equations admitting quasi-local symmetries. Finally, we
generalize the approach in question for the case of an arbitrary system of
evolution equations with two independent variables.
