Image copyright AP Image caption Four recent North Korean missile launches - such as this one last month - are said to have failed

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 150km (90 miles) before landing in the sea.

But the second, launched hours later, flew about 400km and reached an altitude of 1,000km, 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.

In January it conducted its fourth nuclear test, claiming it to be its first using a hydrogen bomb. Shortly after that Pyongyang launched a satellite, widely seen as a test of long-range missile technology.

South Korea has held a national security meeting to discuss Wednesday's missile launches, the Yonhap news agency reported.

Analysis: Stephen Evans, BBC News, Seoul

Image copyright EPA Image caption North Korea has never conducted a full flight test of a Musudan missile

North Korea has tested these medium-range missiles four times now in three months and twice on Wednesday. Outside experts and intelligence agencies believe the first five launches were certainly failures, but aren't sure about the sixth.

It seems to have gone about 400km, far short of its maximum range. That may be because it failed or it may be because a decision was taken not to send it over Japan which had said it would shoot any missile down.

So why is North Korea conducting tests so frequently?

Some Western scientists say that the more conventional way of testing is to conduct the test, and if it fails, to go away and work out why before trying again some months or even a year later.

If this steady method over a long period isn't being followed by North Korea, it may be because of intense pressure from the top, a pressure the scientists on the ground will no doubt feel.

'Provocative actions'

The US State Department has strongly condemned the launches, with spokesman John Kirby saying they would only increase international efforts to stop North Korea's weapons programme.

"We intend to raise our concerns at the UN to bolster international resolve in holding [North Korea] accountable for these provocative actions," said Mr Kirby in a statement.

North American Defense Command (Norad), which tracked the missiles, determined they did not pose a threat to North American territories.

Image copyright Reuters Image caption South Korea's presidential office has announced that it would hold a national security meeting to discuss the launch

Nato Secretary General Jens Stoltenberg said the launches "undermine international security and dialogue".

Japanese Prime Minister Shinzo Abe has said such tests "clearly cannot be tolerated".

The Musudan is believed to have a range of about 3,000km - far enough for it to hit South Korea, Japan and the US territory of Guam in the Western Pacific.

North Korea is thought to have dozens of Musudans but has never conducted a full-distance test.

The four other missiles tested in the last two months either exploded mid-air or crashed.

Surrounding countries had detected preparations for a launch in the past few days and warned that it was about to happen.

The Musudan missile

The Musudan, also known as the Nodong-B or the Taepodong-X is an intermediate-range ballistic missile.

Estimates differ dramatically on its range, with Israeli intelligence putting it at 2,500km and the US Missile Defense Agency estimating about 3,200km. Other sources put its upper limit at 4,000km.

The lower range of the Musudan will enable it to hit the whole of South Korea and Japan. At its upper range it would be able to target US military bases on Guam.