Researchers at the Middlebury Institute of International Studies and the satellite operator Planet Labs Inc. told the news outlet that satellite images taken of North Korea's eastern port city of Sinpo may suggest that the Asian country is mobilising efforts to launch a submarine capable of carrying ballistic missiles.
The images captured the construction of a large structure jutting out into the water from the quay. Images taken in early September were compared with the latest ones from Thursday, showing that the structure had grown to about 100 metres (328 feet) in that time.
According to the researchers, North Korea's conventional submarines are usually built-in structures much smaller than the one their satellites captured images of.
The structure could be used for the construction and maintenance of North Korea's submarine-launched ballistic missile programme, a senior researcher said.