North Korea on Thursday made two failed bids to test fire a powerful, new medium-range ballistic missile, in a thwarted display of military strength ahead of a landmark ruling party congress.

South Korea's defence ministry said an initial morning launch of what was understood to be a Musudan missile saw the rocket plunge back to earth seconds after take-off.

A second attempt in the evening -- again of a Musudan -- also appeared to have failed, a ministry official said.

North Korea has now made three unsuccessful bids in two weeks to test-fly a Musudan, which is capable of striking US bases on the Pacific island of Guam.

The first effort on April 15 -- the birthday of founding leader Kim Il-Sung -- ended in what the Pentagon described as "fiery, catastrophic" failure, with the missile apparently exploding just after take-off.

South Korean military officials say the North is desperate to register a successful launch ahead of next week's party congress, at which leader Kim Jong-Un is expected to take credit for pushing the country's nuclear weapons programme to new heights.

- Nuclear test fears -

There is growing concern that Pyongyang is also preparing to conduct a fifth nuclear test before the party gathering begins on May 6.

In recent months the North has claimed a series of major technical breakthroughs in developing what it sees as the ultimate goal of its nuclear drive -- an intercontinental ballistic missile capable of delivering a nuclear warhead to targets across the continental United States.

The achievements trumpeted by Pyongyang have 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.

Last Saturday, it successfully tested a submarine-launched ballistic missile (SLBM) and was promptly criticised by the UN Security Council.

Existing UN resolutions forbid North Korea from the use of any ballistic missile-related technology, and South Korea said it would push for fresh penalties to be imposed on Pyongyang.

"The government strongly condemns this additional ballistic missile launch... which is a clear violation of UN resolutions and an act of provocation," the foreign ministry in Seoul said after the failed Musudan test on Thursday morning.

It said it would work with other UN members to "try to put the North's regime under more sanctions."

- Longer range -

The Musudan is believed to have 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.

The missile has never been successfully flight-tested.

Three failures in swift succession will be seen as an embarrassment for the leadership, especially as it has built up the party congress -- the first to be held for nearly four decades -- as an opportunity to celebrate the country's achievements.

Speaking last weekend during a visit to Germany, US President Barack Obama warned that North Korea was making dangerous progress even when its efforts fell short of outright success.

"Although, more often than not, they fail in many of these tests, they gain knowledge each time," Obama said.

"We take it very seriously, so do our allies and so does the entire world," he added.

Anxiety has been high on the divided Korean peninsula since Pyongyang conducted its fourth nuclear test in January and a rocket launch a month later that was widely seen as a disguised ballistic missile test.

The UN Security Council responded with its toughest sanctions to date, angering the North, which has since made repeated threats of attacks targeting the South and the United States.