The launch, which South Korean officials confirmed about two hours after an eight-day launch window opened this morning, follows North Korea's widely disputed claim last month to have tested a hydrogen bomb.
It will be considered a further provocation by Washington and its allies and likely draw more sanctions and condemnation from the United Nations.
North Korean rocket and nuclear tests are seen as crucial steps toward the North's ultimate goal of a nuclear armed long-range missile arsenal. North Korea says its nuclear and missile programs are necessary to defend itself against what it calls decades of US hostility.
The January 6 nuclear test has led to another push in the UN to tighten sanctions. North Korea in 2013 also did a nuclear test and then unnerved the international community by orchestrating an escalating campaign of bombast, including threats to fire nuclear missiles at the US and Seoul.
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North Korea has spent decades trying to develop operational nuclear weapons.
It is thought to have a small arsenal of atomic bombs and an impressive array of short- and medium-range missiles. But it has yet to demonstrate that it can produce nuclear bombs small enough to place on a missile, or missiles that can reliably deliver their bombs to faraway targets.
Still, the North's nuclear tests and steadily improving long-range rocket launches push its nuclear aims further along.