In this study, three magnetometer stations in China and four co-located magnetometers and geoelectric field detectors in Japan were primarily used for observing co-seismic signatures excited by nuclear explosions conducted in North Korea between 2006 - 2017. The observations in Japan did not measure considerable magnitudes of the co-seismic electromagnetic (EM) signals corresponding to the geomagnetic and geoelectric fields at large distances from the explosion source. However, the geomagnetic field detectors in northeast China detected clear co-seismic signatures, shortly after the arrival time of the seismic waves. On the day of the most powerful blast on 3 September 2017, the horizontal component of the geomagnetic field with a maximum amplitude of ±5 nT was measured at the Changchun station in China, located at a distance of less than 500 km from the explosion source. In order to understand the measured signals, three mechanisms that can induce these signals were simulated. The electrokinetic and dynamo effects were observed to primarily contribute to the formation of the surface-wave-related EM signals, whereas the shaking and vibration of the instrument caused the continuous oscillation seen in the geomagnetic observations.