UML2.X sequence diagrams (SD) are equipped with high structures: the combined fragments (CF) that permit to model complex behaviours of systems. CF can be nested to allow more sophisticated behaviours, however they complicate the interpretation of the SD and the computation of precedence relations between the events. In a previous work, we proposed a causal semantics for UML2.X SD. It is based partial order theory, its well-defined relations allow the computation of all precedence relations for the events of UML2.X SD with nested CF. We considered the most popular CF of control-flow alt, opt, loop, seq allowing to model respectively alternative, optional, iterative and sequential behaviours. In this work, we improve that previous work to consider a par CF allowing to model parallel behaviours, and we propose an operational semantics that is based on the causal semantics. The proposed operational semantics is a substantial step towards the refinement checking and the analysis of some properties of SD.