North Korea has launched what appears to be a second intermediate-range Musudan missile.
It flew about 400km in what would be its most effective test yet, hours after another launch failed, South Korea's military said on Wednesday.
It was not immediately clear if the second Musudan launch, about two hours after the first, was considered a success or failure, or how the flight ended.
However, the distance it covered was theoretically more than halfway towards the southwest coast of Japan's main Honshu island.
The missile reached an altitude of 1000km, indicating North Korea had made progress in its missile programs, Japan's Minister of Defence Gen Nakatani said.
"The threat to Japan is intensifying," Nakatani told reporters in Tokyo.
The first missile was launched from the east coast city of Wonsan, a South Korean official said, the same area where previous tests of intermediate-range missiles were conducted, possibly using mobile launchers.
South Korea's Yonhap news agency, quoting a government official, said it disintegrated mid-air after a flight of about 150km.
The launches were in continued defiance of international warnings and a series of UN Security Council resolutions that ban the North from using ballistic missile technology, which Pyongyang rejects as an infringement of its sovereignty.
Wednesday's first launch would have been the fifth straight unsuccessful attempt in the past two months to launch a missile that is designed to fly more than 3000km and could theoretically reach any part of Japan and the US territory of Guam.
Nakatani said North Korea's repeated missile launches were a "serious provocation" and could not be tolerated.
Japan indicated after the first launch that it would protest strongly because it violated a UN resolution, even though the launches posed no immediate threat to Japanese security.
In Seoul, South Korea's presidential office said a national security meeting would be convened later on Wednesday to discuss the latest missile launches.
The US military detected the two missiles, most likely Musudan, from North Korea, the US military's Pacific Command said. A Pentagon spokesman said both missiles fell into the Sea of Japan.
Yonhap, citing an unidentified government source, said on Tuesday the North had been seen moving an intermediate-range missile to its east coast. Japan put its military on alert in response.
North Korea is believed to have up to 30 Musudan missiles, according to South Korean media, which officials said were first deployed around 2007, although the North had never attempted to test-fire them until April.
The UN Security Council, backed by the North's main diplomatic ally, China, imposed tough new sanctions in March after the isolated state conducted its fourth nuclear test in January and launched a long-range rocket that put an object into space orbit.
North Korea has conducted a series of tests since then that it claimed showed progress in nuclear weapons and long-range ballistic missile capabilities, including new rocket engines and simulated atmospheric re-entry.