North Korea conducted two back-to-back tests of a powerful new medium-range missile on Wednesday, with one launch ending in failure but the other flying a distance of 400 kilometres (250 miles), South Korea`s Defence Ministry said.
Both tests were believed to be of a much-hyped, intermediate-range Musudan missile capable of reaching US bases as far away as Guam.
The US State Department strongly condemned the launches, saying they represented clear violations of UN resolutions banning North Korea from any use of ballistic missile technology.
The first test shortly before 6:00 am (2100 GMT Tuesday) was deemed to have failed, but the ministry in Seoul said the second Musudan -- fired from the same east coast location two hours later -- had flown 400 kilometres into the East Sea (Sea of Japan).
"South Korea and the United States are conducting further analysis," the ministry said in a statement that stopped short of labelling the second test a success.
After four failed Musudan launches earlier this year, a successful test would mark a major step forward for a weapons programme that ultimately aspires to develop a proven nuclear strike capability against the US mainland.
US State Department spokesman John Kirby said the latest tests would only increase global efforts to counter North Korea`s illicit weapons programme."We intend to raise our concerns at the UN to bolster international resolve in holding (North Korea) accountable for these provocative actions," Kirby said in a statement.
Japanese broadcaster NHK quoted Prime Minister Shinzo Abe as saying such tests "cannot be tolerated".
The Musudan has an estimated range of anywhere between 2,500 and 4,000 kilometres (1,550 to 2,500 miles). The lower range covers the whole of South Korea and Japan, while the upper range would include US military bases on Guam.
First unveiled as an indigenous missile at a military parade in Pyongyang in October 2010, the Musudan has never been fully flight-tested.
Three failures in April were seen as an embarrassment for North Korea`s leadership, coming ahead of a rare ruling party congress that was meant to celebrate the country`s achievements.
Another attempt in May was also deemed to have failed.Wednesday`s tests came with military tensions still running high following Pyongyang`s fourth nuclear test in January and a long-range rocket launch a month later that saw the UN Security Council impose its toughest sanctions to date on the North.
During the party congress in May, North Korean leader Kim Jong-Un had personally extended an offer of military dialogue with the South.
The proposal was repeated several times by the North`s military, but Seoul dismissed all the overtures as insincere "posturing" given Kim`s vow at the same congress to push ahead with the country`s nuclear weapons programme.
In recent months, North Korea has claimed a series of technical breakthroughs in developing an intercontinental ballistic missile (ICBM) capable of delivering a nuclear warhead to targets across the continental United States.
The claimed achievements included miniaturising a nuclear warhead to fit on a missile, developing a warhead that can withstand atmospheric re-entry and building a solid-fuel missile engine.
The North also hailed the successful test of an engine specifically designed for an ICBM that would "guarantee" an eventual nuclear strike on the US mainland.
Outside experts have treated a number of the claims with scepticism, while acknowledging that the North has made significant strides in upgrading its nuclear arsenal.