UML has become a de-facto standard for design and development of object-oriented systems. On the other hand, UML includes various diagrams and notations which are missing of formal semantics. There does not exist much work to transform completely UML models to executable codes by any of the computer-aided software engineering tools because of the hidden semantics under the UML diagrams. Moreover, UML models cannot be used for automated analysis for validation and verification of the systems. Therefore, an integration of UML and semantics-based techniques such as formal methods is required to overcome such type of issues. Further, proof of correctness of software models plays a key role to ensure the quality of the final system to be developed. This paper presents a systematic method for transformation and verification of UML sequence diagrams into Z specification. The method works by translating and analysing few important combined fragments of sequence diagrams using Z notation. At first, formal specification of primary constructs is provided by capturing the hidden semantics under the diagram. Formal analysis of interaction operators namely options, alternatives, loops and break of the fragments is provided. The combined fragments are important because it allows multiple scenarios to increase an expressive power of sequence diagram by various control flows. Finally, formal analysis is provided by Z/Eves tool to check the syntax, type and proof for consistency and correctness of the specification.