North Korea apparently failed with an attempted missile launch on Tuesday, the latest in a series of setbacks for a ballistic weapons programme that aspires to threaten the US mainland.
South Korea's defence ministry detected the dawn launch effort, which Japan condemned as an unacceptable and "provocative" act.
The ministry declined to speculate on the missile type, but military sources cited by local media said it was a powerful, medium-range "Musudan" that has already undergone three failed launches this year.
UN resolutions ban North Korea from any use of ballistic missile technology, although it regularly fires short-range missiles into the sea off its east coast.
Tuesday's effort came with tensions still running high on the divided Korean peninsula following the North's fourth nuclear test in January and long-range rocket launch a month later.
"We believe that it was a failure," said Jeon Ha-Gyu, spokesman for the South Korean Joint Chiefs of Staff.
"As to why and how it failed, we are in the process of analysing that," Jeon told a press briefing.
"We are maintaining a strong defence posture with potential further provocations by the North in mind," he added.
String of failures 
In April, the North failed three times to test fire a Musudan, which has an estimated range of anywhere between 2,500 and 4,000 kilometres.
The lower range covers the whole of South Korea and Japan, while the upper range would include US military bases on Guam.
"North Korea's repeated ballistic missile launches are serious, provocative acts against the international community, including Japan," Japanese Foreign Minister Fumio Kishida told a regular press briefing.
"We absolutely cannot accept this," Kishida said.
First unveiled as an indigenous missile at a military parade in Pyongyang in October 2010, the Musudan has never been successfully flight-tested.
The three failures in April were seen as an embarrassment for the Pyongyang leadership, coming ahead of a ruling party congress in May that was meant to celebrate the country's achievements.
South Korea's Yonhap news agency quoted official sources as saying Tuesday's missile may have exploded on its mobile launcher.
"The explosion is presumed to have inflicted serious injuries on personnel in the immediate vicinity," Yonhap said.
Talks offer 
During the party congress, North Korean leader Kim Jong-Un personally extended an offer of military dialogue with the South aimed at easing tensions.
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 what it sees as the ultimate goal of that programme -- an intercontinental ballistic missile (ICBM) capable of delivering a nuclear warhead to targets across the continental United States.
They 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.
It 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.