US researchers say satellite images suggest North Korea may be preparing to deploy a ballistic missile-capable submarine.
Researchers at the Middlebury Institute of International Studies and satellite operator Planet Labs have told NHK that they have analyzed images of a shipyard in the eastern port city of Sinpo.
On September 3 at least 13 pillar-like objects could be seen along a quay, with some vehicles moving around.
Images taken on September 12 show the construction of a large structure straddling the quay and the water.
The most recent images from Thursday, show the structure covering almost the entire quay.
The researchers say the structure is now about 100 meters long, which is much longer than what is used to build North Korea's conventional submarines.
They say Pyongyang may be preparing to launch a new submarine capable of carrying ballistic missiles.
Senior Research Associate David Schmerler says the structure could be used for the maintenance and development of submarines, and drive the country's submarine-launched ballistic missile program.