The article is devoted some important issues of the functional safety analysis, in particular the safety integrity level (SIL) verification of safety functions to be implemented within the distributed control and protection systems with regard to cyber security aspects. The procedure for functional safety management includes hazard identification, risk analysis and assessment, specification of overall safety requirements and definition of safety functions. Based on risk assessment results the safety integrity level (SIL) is determined for consecutive safety function. These functions are implemented within industrial control system (ICS) that consist of the basic process control system (BPCS) and/or safety instrumented system (SIS). Determination of required SIL related to required risk mitigation is based on semi-quantitative evaluation method. Verification of SIL for considered architectures of BPCS and/or SIS is supported by probabilistic models with appropriate data and model parameters including cyber security-related and uncertainty aspects. A method based on quantitative and qualitative information is proposed for SIL verification with regard of the evaluation assurance levels (EAL), the security assurance levels (SAL) and the number of protection rings described in the SeSa methodology. A method for SIL verification, based on so called differential factor is presented. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.