Despite the widespread adoption of UML as a standard for modeling software systems, it does not provide adequate support for specifying User Interface (UI) requirements. It has become a common practice to separately use UML use cases for specifying functional requirements and task models for modeling UI requirements. The lack of integration of these two related models is likely to introduce redundancies and inconsistencies into the software development process. In this paper, we propose an integrated model, consisting of use case and task models, for capturing functional and UI requirements. Both artifacts are used in a complementary manner and are formally related through so-called Anchors. Anchors are use case steps that require further elaboration with UI-specific interactions. These interactions are explicitly captured in associated task models. The formal semantics of the integrated model is given with finite state automata.