UML and concurrency

Is there any way (official according to UML 2.0) to have sequence diagrams or activity diagrams in multithreaded environment? Or UML is just "sin开发者_JS百科glethreaded"?

In UML Activity Diagrams you can fork the flow of control to more than one stream which will be executed simultaneously and you can join them again to unite (maybe before the end of the activity).

In sequence diagram, you can define block/operator (i dont know the proper english term) par to denote, that some actions are parallel...almost every relevant diagram has its own way to define parallelism.