A Japanese daily says a group of North Korean nuclear and missile technicians visited Iran shortly after Pyongyang’s April fifth rocket launch.

Quoting U.S. and Japanese diplomatic sources, the Sankei Shimbun said Tuesday that some 50 North Korean technicians visited a region in central Iran suspected of housing an underground nuclear facility. The daily said attention is being drawn to whether the visit is connected to the North’s nuclear test on Monday morning.

The daily added that the North, which has been seeking to enhance its development of nuclear weapons based on plutonium, may be attempting to acquire technology from Iran regarding uranium enrichment, particularly on centrifugal separation.

The report said Pyongyang invited a dozen military officials and scientists from Iran to North Korea around the time of its April fifth rocket launch to exchange information on rocket technology.