North Korean leader Kim Jong Un has held a ceremony to reveal a new submarine, which Pyongyang claims can carry nuclear weapons.
A report in state media said the submarine strengthened the country's nuclear deterrent by "leaps and bounds".
A nuclear-attack submarine has long been on the list of weapons North Korea wants to build.
In photographs released by state media, Mr Kim is standing in a shipyard, surrounded by naval officers, and overshadowed by an enormous black submarine, which the report claimed is capable of launching nuclear weapons.
Mr Kim has been working to develop this submarine for many years, and it is seen as a crucial part of his nuclear weapons programme.
Submarines are difficult to locate under water, meaning having one could allow Pyongyang to strike its enemies from the sea, after its land weapons had been destroyed in an attack.
But we don't know if this submarine is operational. The North Koreans have yet to demonstrate it can successfully fire nuclear-capable missiles.