Web services are an interesting emerging technology for the development of distributed applications with respect to SOA (Service Oriented Architecture) paradigm due to the many features provided by the technology. One of these features is service composition, which consists of combining existing services to provide a richer new composite service to meet some user requirements. This paper proposes a new method for composing web services by using UML scenarios expressed as sequence diagrams, and an MDA (Model Driven Architecture) approach to generate the code of the composite service in BPEL (Business Process Execution Language). Our approach consists of three main steps. In the first step, existing simple web services are discovered and located in the web service registry. The developer imports the WSDL files of the candidate services and translates them into UML diagrams (class diagram, use case diagram and sequence diagrams). In the second step, the sequence diagrams, referred to as scenarios, are integrated into a single sequence diagram that describes the behaviour of the composite web service, by using the interaction operators of UML 2.0. In the third step, the MDA approach is adopted to transform the resulting sequence diagram into a BPEL process. As such, our method has the advantages of being independent of the web service composition language and the UML modelling tool, the user can choose any UML design tool, any language of composition and any execution engine to compose services.