Forming the requirements catalogue and having elicited and analysed your audiences requirements up to the point of formal signoff, based on the needs of a project you UML diagrams would normally make reference to a working of functionality or a process which you captured, process workflows/activity diagrams are sound for stakeholders to visualise how there internal processes work, use cases, class diagrams, sequence diagrams etc are more useful for development and QA teams during the design/development phases for devs and qa's.
For each requirement just make reference to the UML diagrams through the use of the requirements ID. But as mentioned earlier, ensure you have traceability between requirements and UML and vice versa,
Hope this helps