Frontline nurses' burnout and its associated factors during the COVID-19 pandemic in South Korea

•Nurses working in a high-risk environment had higher burnout levels.•Nurses scored higher on the client-related burnout scale.•Assigned number of patients, insomnia, and depression mainly affected burnout.•Professional and psychological factors should be considered to ameliorate burnout.•Burnout-focused interventions may promote psychological well-being of HCWs.