North Korea has test-fired two mid-range ballistic missiles from its eastern coast, says South Korea.
The first launch was considered to have failed, travelling about 150 km before landing in the sea.
But the second, launched hours later, flew about 400 km and reached an altitude of 1,000 km, the most effective test to date.
A confirmed successful test would mark a step forward for North Korea after four failed launches in recent months.
Both launches are believed to have been intermediate-range Musudan missiles.
South Korea's Joint Chiefs of Staff said Seoul and the US were "carrying out an in-depth analysis" of the second missile, and did not say whether it was considered a success.
North Korea, which is developing nuclear weapons, is banned by UN resolutions from any use of ballistic missile technology.