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The damping of inter-area oscillations is a major concern for many power system operators. This paper presents a novel method for monitoring of inter-area oscillations based on system identification using subspace techniques and modal analysis. The method enables the detection of oscillatory modes as well as the damping and frequency of those from ambient data recorded during normal operation of the power system. Through modal observability which is also estimated, the parts of the grid participating in an oscillation mode can be determined. The method is suitable for real-time implementation. The paper demonstrates application of the method to measurements from the Nordic power system and to synthetic data. The technique can be used to increase operator situational awareness and provide early warning in case of potential for unstable inter-area mode oscillations.