Context: Certification of safety-critical systems (SCS) demands thorough documentation that demonstrates why a system shall be considered safe. Assurance Case Development (ACD) is an approach for discussing, analyzing and assessing the safety properties of systems. Software requirements of a SCS are an essential information included in assurance cases, alongside system design and safety analysis information. Lack of integrated analysis of requirements and safety concerns may lead to safety issues in the development of critical systems. One of the challenges for the agile development of SCS is to address both Requirements Engineering (RE) and ACD in an integrated way throughout the development life cycle. Objective: This research proposes a framework to integrate the Assurance Case Development and Agile RE in the development of SCS. Method: This research is organized in three main phases. First, a systematic mapping study is performed to understand how incremental ACD is addressed by current approaches, and a survey with experts is carried out to understand the development of ACs during Agile RE activities in practice. Second, the framework and its supporting tools and documentation will be designed and developed. Finally, a series of empirical studies will evaluate aspects of the framework in a multi-perspective manner and as long as it is developed. Conclusions: We expect that this approach contributes to leverage the development of ACs earlier and integrated with RE activities in agile development of SCS.