Context: Safety-Critical Systems (SCS) require more sophisticated requirements engineering (RE) approaches as inadequate, incomplete or misunderstood requirements have been recognized as a major cause in many accidents and safety-related catastrophes. Objective: In order to cope with the complexity of specifying SCS by RE, we investigate the approaches proposed to improve the communication or integration between RE and safety engineering in SCS development. We analyze the activities that should be performed by RE during safety analysis, the hazard/safety techniques it could use, the relationships between safety information that it should specify, the tools to support safety analysis as well as integration benefits between these areas. Method: We use a Systematic Literature Review (SLR) as the basis for our work. Results: We developed four taxonomies to help RE during specification of SCS that classify: techniques used in (1) hazard analysis; (2) safety analysis; (3) safety-related information and (4) a detailed set of information regarding hazards specification. Conclusions: This paper is a step towards developing a body of knowledge in safety concerns necessary to RE in the specification of SCS that is derived from a large-scale SLR. We believe the results will benefit both researchers and practitioners.