SEOUL: North Korea test-fired a submarine–launched ballistic missile on Wednesday, days after threatening a nuclear strike in retaliation at the start of large-scale South Korea-US military exercises.
A statement from South Korea's military Joint Chiefs of Staff said the missile, launched in the early morning from a submarine in the East Sea (Sea of Japan), flew around 500 kilometres (310 miles) -- a substantial improvement on similar tests in the past.
The launch came amid escalating cross-border tensions and after tens of thousands of South Korean and US troops kicked off their annual "Ulchi Freedom" military drill on Monday.
The Joint Chiefs of staff said Wednesday's test was clearly aimed at fuelling those tensions and was a "serious challenge" to security on the Korean peninsula, as well as a grave breach of UN resolutions.
"We will deal strongly and sternly with any provocation by the North," the statement said.
Seoul and Washington insist their annual joint military exercises are purely defensive in nature, but Pyongyang views them as wilfully provocative.
North Korea on Monday condemned the Ulchi Freedom drill as an "unpardonable criminal act" and warned that any violation of territorial sovereignty would result in a "pre-emptive nuclear strike".
Pyongyang has repeatedly threatened similar attacks against the South and US targets there and elsewhere, although the main focus of its nuclear weapons programme is to develop a credible strike threat against the US mainland.
The Ulchi Freedom drill plays out a scenario of full-scale invasion by the nuclear-armed North. It is largely computer-simulated but still involves around 50,000 Korean and 25,000 US soldiers.
The exercise always triggers a rise in tensions on the divided peninsula, and this year it coincides with particularly volatile cross-border relations following a series of high-profile defections.
Last week North Korea's deputy ambassador to Britain, Thae Yong-Ho, defected to the South -- a rare and damaging loss of diplomatic face for Pyongyang and a major PR victory for Seoul.
In comments clearly aimed at riling Pyongyang, South Korean President Park Geun-Hye said such high-ranking defections suggested "serious cracks" in supreme leader Kim Jong-Un's regime.
Park also warned that North Korea may carry out "various terror attacks and provocations" in a show of strength aimed at building national unity and loyalty to Kim.
On Sunday, the Unification Ministry in Seoul urged all citizens to be on guard against possible North Korean assassination attempts on defectors and anti-Pyongyang activists in the South.
North Korea has conducted a number of submarine-launched ballistic missile (SLBM) tests -- most recently in April and July -- with varying degrees of success.
Previous flight distances have not exceeded 30 kilometres, and the Joint Chiefs of Staff statement acknowledged that Wednesday's test showed a marked improvement.
The South Korean Defence Ministry has said the North could be able to deploy a working SLBM within three to four years.
A proven SLBM capability would take North Korea's nuclear strike threat to a new level, allowing deployment far beyond the Korean peninsula and the potential to retaliate in the event of a nuclear attack.
Current UN resolutions prohibit North Korea from any test of ballistic missile technology, but Pyongyang has continued to carry out numerous launches following its fourth nuclear test in January.
South Korea responded by agreeing to deploy a sophisticated US anti-missile system -- a move that has seriously strained relations with North Korea's main diplomatic ally, China.