S. Korea Tells North to Halt 'Worrisome' Sub Missile Project

South Korea on Monday urged North Korea to halt its "worrisome" development of a submarine-launched ballistic missile, and threatened to respond "mercilessly" to any overt provocation from Pyongyang.

The North's state media announced Saturday that a new SLBM had been tested under the personal supervision of North Korean leader Kim Jong-Un, who described it as an "eye-opening success" that provided his military with a "world-level strategic weapon".

The exact nature of the test has yet to be independently confirmed, with some experts suggesting it was an ejection test -- with the missile only travelling a few hundred meters -- rather than a full flight test.

However, South Korea's defense ministry said the test was "very serious and worrisome" and could undermine regional stability.

"We urge the North to immediately stop development of the SLBM that threatens the security of the Korean peninsula and northeast Asia," ministry spokesman Kim Min-Seok told reporters.

A fully developed SLBM capability would take the North Korean nuclear threat to a new level, allowing deployment far beyond the Korean peninsula and the potential to retaliate in the event of a nuclear attack.

The main strategic threat from such a weapon is the difficulty in tracking a submerged, armed submarine, Kim said. 

South Korea's assessment after the test -- which was believed to have taken place Friday -- was that the North is still in the "early phase" of development.

"In advanced countries, it took about four to five years to develop operational SLBMs," Kim said.

According to the South Korean military, the North followed up the SLBM launch by test firing three anti-ship cruise missiles Saturday, on the back of threats to target South Korean naval boats Pyongyang accuses of violating their disputed maritime border.

In a security meeting with ruling Saenuri party officials in Seoul, Defense Minister Han Min-Koo said the South's military would not sit idly by in the face of North Korean aggression.

"We will mercilessly retaliate to break their cycle of provocation," Han was quoted as saying by the Yonhap news agency.

"Retaliation against provocation is an order from the people," Han said.

Pyongyang has issued three warnings over the past week that it will fire on sight at South Korean navy patrol boats it says are routinely entering its territorial waters in the Yellow Sea.

Seoul has denied any incursions.