1. A method of generating annotated data for use in a natural language understanding (NLU) system, the method comprising: parsing, by a computer device, an input annotated phrase having semantic labels; generating a syntactic tree reflecting a grammatical structure of the phrase parsed; generating one or more annotated alternative versions of the input annotated phrase based on the generated syntactic tree in a manner preserving alignment between expressions and corresponding semantic labels in the input annotated phrase in the one or more annotated alternative versions generated; and training a NLU system with the generated one or more annotated alternative versions of the input annotated phrase.