- Pyongyang test fired two ballistic missiles on Wednesday, making six launches since April
- Launches are the latest in North Korea's efforts towards a long-range missile programme capable of striking the US mainland
- The US has condemned the launches as a clear violation of UN resolutions banning North Korea from using ballistic missile technology
- UN sanctions on North Korea were tightened in January after it conducted its fourth nuclear test
North Korea has conducted back-to-back tests of a mid-range ballistic missile from its eastern coast, South Korean officials say.
One test of the powerful Musudan intermediate-range missile made considerable improvements in flight distance and altitude over previous failed launches, but still fell well short of the missile's projected range.
Wednesday's tests marked Pyongyang's fifth and sixth attempt since April to successfully launch its Musudan missile.
"South Korean defense officials didn’t want to label it as either a success or a failure, but that does appear to be the most effective test of this Musadan intermediate-range missile so far even though it did fall far short of its expected range," said Al Jazeera's correspondent in Beijing Florence Looi.
The first five launches failed, either exploding in midair or crashing, and the sixth only flew about 400km - well short of the missile’s 3,500-km potential and not long enough to be classified as intermediate.
One of the missiles launched on Wednesday reached an altitude of more than 1,000km, displaying marked progress in comparison with previous attempts, Japan's defence minister said on Wednesday.
"We don't know whether it counts as a success, but North Korea has shown some capability with IRBMs," Gen Nakatani told reporters in Tokyo. "The threat to Japan is intensifying."
The ongoing tests, apparently linked to an order made by North Korean leader Kim Jong-un in March, show the North's determination to pursue its goal of a nuclear missile programme capable of threatening the United States mainland.
Despite the repeated failures, North Korea's persistence in testing the Musudan missile has worried Washington and its allies, Tokyo and Seoul. The missile's range puts a large part of Asia and the Pacific, including US military bases, within reach.
The US State Department condemned the launches on Wednesday, saying that they represented blatant violations of United Nations resolutions banning nuclear-armed North Korea from any use of ballistic missile technology.
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"We intend to raise our concerns at the UN to bolster international resolve in holding [North Korea] accountable for the provocative actions," State Department spokesman John Kirby said in a statement.
And while US and North Korean diplomats were due to attend a six-nation security forum in Beijing on Wednesday - a rare opportunity for contact between the two powers - the State Department has said there were no plans for direct talks.
This is the first time in two years that North Korea has attended the six-nation forum, which is attended by representatives from the same nations that attend the six party nuclear talks, now stalled since 2008.
But while there are expectations that Pyongyang's attendance may foreshadow a resumption of the six party talks, the US has mandated that in order for those talks to resume, North Korea must give up its nuclear programme.
"From what we've seen, from Wednesday and from the past six months alone, North Korea seems to be doing exactly the opposite," according to Al Jazeera's Florence Looi.
Japanese Prime Minister Shinzo Abe was quoted by national broadcaster NHK as saying such tests "cannot be tolerated".
South Korea's Unification Ministry called the launches a "clear provocation" that violated UN Security Council resolutions banning any ballistic activities by North Korea.
Pyongyang conducted its fourth nuclear test in January, and later that month launched a long-range rocket that some say was a cover for a test of banned missile technology.
"It claims to have tested its first hydrogen bomb, although this account is disputed, but experts do say they appear to have at least tested some components of a hydrogen bomb," said Al Jazeera's Florence Looi.
"So it does appear that they are accelerating their nuclear program and their ballistic weapons programme. And it has increased tensions, not only on the Korean Peninsula, but also in the region."
The tests in January saw the UN Security Council impose its toughest sanctions to date on the North.
North Korea has claimed a number of technical breakthroughs in recent months in its push to develop an intercontinental ballistic missile capable of delivering a nuclear payload to targets across the continental US. But officials in South Korea say the North does not yet possess such a weapon.
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Recently claimed achievements include the creation of a solid-fuel missile engine, the development of a nuclear warhead capable of withstanding atmospheric re-entry, and the miniaturisation of a nuclear warhead.
Experts outside the country have been sceptical of Pyongyang’s claims, but have acknowledged that the North has made significant gains in improving its nuclear arsenal.
The Korean Peninsula has remained in a technical state of war since the end of the Korean War in 1953, when an armistice brought an end to fighting but a peace treaty was never signed.
The US has some 28,000 soldiers stationed in South Korea to deter any potential aggression from North Korea.
Source: Al Jazeera and agencies