South Korea's Yonhap news agency said in an unsourced report that the missile was a powerful mid-range Musudan, which, if true, would make it the fourth failure by the North to conduct a successful test launch of the new missile, which could potentially reach far-away US military bases in Asia and the Pacific. Seoul defence officials could not immediately confirm the report.
The military is analysing what happened and had no other details.
Despite recent failures, there has been growing outside worry over North Korea's nuclear and missile activity this year, which includes a nuclear test in January and a long-range rocket test in February that outsiders see as a test of banned long-range missile technology.
The most recent launch follows Seoul's rejection of recent Pyongyang overtures to talk, part of what some analysts see as an attempt by the North to start a dialogue meant to win the impoverished country aid.
South Korea believes the North does not yet possess a missile capable of hitting the US mainland, but the North is working on that technology.
Before April's suspected launches, North Korea had never flight-tested a Musudan missile, though one was displayed during a military parade in 2010 in Pyongyang.