North Korea said on Thursday (September 16) that the missiles it fired the day before were a test of its new 'railway-borne missile system', designed as a potential counter-strike to any forces threatening the country.
South Korea and Japan said on Wednesday (September 15) they detected two missiles launched by Pyongyang, just days after a previous test.
When North Korean state media KCNA released photos of the launch, it said the missiles struck a target in the sea off the country's east coast.
It also reported possible plans to expand the North's railway-borne missile unit.
Analysts warn that system could make it more difficult for a foreign military to track and destroy the missiles before they are fired.
Though they also said the system will likely be constrained by the country's relatively limited and sometimes unreliable rail network.
Wednesday's launch also came the same day that Seoul tested a submarine-launched ballistic missile, making South Korea the first country without nuclear weapons to develop such a system.
The two Koreas have been locked in an increasingly-heated arms race that comes amid stalled negotiations to convince Pyongyang to denuclearise.