One of two powerful medium-range missiles launched by North Korea on Wednesday reached an altitude of 1,000 kilometres (620 miles), Japan's defence ministry said, with analysts saying that accomplishment is sufficient to label it a "success".

Both launches were believed to be of a Musudan -- a much-hyped, intermediate-range missile capable of reaching US bases as far away as Guam -- South Korean and Japanese defence officials said.

Though still analysing the result, Japan's "radar information shows the missile reached an altitude of 1,000 kilometres," the defence ministry said in a statement.

"The fact that a missile estimated to be a Musudan was launched and a certain functionality was exhibited is a serious concern," it added.

If the missile did reach 1,000 kilometres, it would have been at around twice the altitude of the Hubble Space Telescope and some 600 kilometres further into space than the International Space Station.

The first missile, fired shortly before 6:00 am (2100 GMT Tuesday), was deemed to have failed after reportedly travelling around 150 kilometres (90 miles) over the Sea of Japan (East Sea).

But the second -- fired from the same east coast location two hours later -- flew 400 kilometres, the South Korean and Japanese ministries said.

Toshimitsu Shigemura, a North Korea expert and now professor emeritus at Waseda University in Tokyo, called the latter launch "a success, as the North launched it vertically rather than obliquely" so as to avoid the airspace and territory of other countries.

The accomplishment, however, does not pose an immediate threat to the West, he stressed.

"It is a slight advance in the Musudan technology but the pace of improvement is slow and it does not pose threat to the United States," Shigemura added.

Melissa Hanham, an expert on North Korea's WMD programme at the Middlebury Institute of International Studies in California, also cited the Japanese data.

"That likely makes this a success," she told AFP.

Existing UN Security Council measures ban North Korea from any use of ballistic missile technology, and the United States and Japan swiftly condemned the launches.