Warning Over North Korea Ballistic Missile Subs

North Korea could have a submarine fleet capable of launching ballistic missiles in two or three years, the South Korean government has warned.

The caution came after North Korea published pictures of its leader Kim Jong-Un watching a missile test launch from a submarine.

Mr Kim hailed the missile as a "world-level strategic weapon capable of striking and wiping out in any waters the hostile forces infringing upon (North Korea's) sovereignty and dignity", said the North's KCNA state news agency.

There was no immediate independent confirmation of the test, which would mark a major breakthrough for the country's missile programme and violate UN resolutions prohibiting Pyongyang from developing or using ballistic missile technology.

North Korea has said it is developing nuclear warheads. Submarine-launched missiles would take its nuclear threat to a new level, allowing attacks far beyond the Korean peninsula.

South Korea's defence 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 (submarine-launched ballistic missile) that threatens the security of the Korean peninsula and northeast Asia," said ministry spokesman Kim Min-Seok.

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

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

However, a defence source said the North could have a fully functioning submarine armed with ballistic missiles in "two to three years".

The North followed up the launch by test firing three cruise missiles on Saturday, amid threats to target South Korean naval boats it accuses of violating its waters.

South Korea's Defence Minister Han Min-Koo said its military would not sit idly by in the face of North Korean aggression.

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

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

Seoul has denied any incursions.

China, North Korea's only major ally, described tensions on the peninsula as "very complex and sensitive."

"We hope that all relevant parties can do more to ameliorate the tense situation, and jointly maintain peace and stability," said ministry spokeswoman Hua Chunying.

North Korea, which is under heavy sanctions for its missile and nuclear tests, is technically still at war with the South after their 1950-53 conflict ended in a truce, not a peace treaty.