North Korea says it conducted a "crucial" test at a missile launch site on Friday without specifying what it was.
State media outlet KCNA said the test has boosted the country's strategic nuclear deterrent.
South Korea's defence minister said it was an engine test.
A previous test at the Sohae Satellite Launching Station was announced just last week on December 7th.
Both tests appear to be a provocation as they come just ahead of an end of year deadline North Korea has given the United States.
That urges Washington to cease calling for Pyongyang to disarm its nuclear weapons.
But the US has so far refused and it's also not lifting harsh sanctions against the communist regime.