We propose an approach to validate object-based specifications by checking that sequence diagrams can be executed in the B world and do not conflict with safety and dynamic properties. The process begins by a UML specification in the form of a class diagram and sequence diagrams which express scenarios modelling the system's behaviour. These diagrams are transformed into a B specification which is completed by the definition of the operations (messages in the sequence diagrams corresponding to the methods in the class diagram), safety and dynamic properties on the system. The validation of scenarios and the satisfaction of the properties is done by means of a theorem prover. The approach is illustrated on a simplified case study: the access control of persons to a building.