This paper describes a fully automatic technique for verifying properties of higher-order functional programs. Functional programs are modelled with term rewriting systems and tree automata are used to model reachable program states. From a tree automaton representing the initial state, it is possible to define a completion algorithm on tree automata for approximating the output set of the program to verify. We define a subclass of higher-order functional programs for which the completion is guaranteed to terminate. Furthermore, since precision of the completion is fixed by a set of equations, we also propose an algorithm to automatically generate sets of equations by iterative refinement. We present some experiments showing that the result...