North Korean leader Kim Jong Un has ordered further nuclear tests, state media said yesterday, as military tensions surge on the Korean peninsula.
Pyongyang has reacted to large-scale joint exercises between South Korean and United States forces.
Since the joint drills began on Monday, the North has issued daily warnings and statements, talking up its nuclear strike capabilities and threatening to turn Seoul and Washington into "flames and ashes".
Just days after he was photographed posing in front of what state media described as a miniaturised nuclear warhead, Kim said the weapon required further testing.
Overseeing a ballistic missile launch on Thursday, Kim ordered "more nuclear explosion tests to estimate the destructive power of the newly produced nuclear warheads", the North's official KCNA news agency said.
Experts are divided as to just how far the North may have gone in shrinking warheads to a size capable of fitting on a ballistic missile - a major step forward in strike capability that would present a heightened threat to South Korea, other countries in the region and, eventually, the US.
According to KCNA, Thursday's launch of two short-range ballistic missiles, which traversed the eastern part of the country before falling into the East Sea (Sea of Japan), was part of a nuclear strike exercise.
The aim was to simulate conditions for "exploding nuclear warheads from the preset altitude above targets in the ports under enemy control," the agency said.
Watching the exercise, Kim reiterated an earlier threat to launch an immediate nuclear attack if the "sabre-rattling" South Korea-US drills should harm "even a single tree or a blade of grass" on North Korean territory.
"I will issue a prompt order to launch attack with all military strike means," he said.
Military tensions on the divided Korean peninsula have been on the rise since the North carried out its fourth nuclear test in January, followed by a long-range rocket launch last month.
South Korea and the US responded by scaling up their annual joint drills, which Pyongyang has always condemned as provocative rehearsals for invasion.
The North's anger has been fuelled this year by reports that the drills included a "decapitation strike" scenario in which the North Korean leadership and command structure is taken out at the start of any conflict.
In light of such drills, "our self-defensive countermeasures should adopt a more pre-emptive and offensive mode", Kim said.
The United Nations Security Council responded to the North's latest nuclear test and rocket launch by adopting tough, new sanctions, which Pyongyang condemned as a "gangster-like" provocation orchestrated by the US.
Also on Thursday, the North's Committee for the Peaceful Reunification of Korea said North Korea would "liquidate" South Korean assets at the closed factory park in the North Korean border town of Kaesong and at a scrapped tourism resort at Diamond Mountain. It said North Korea would also impose "lethal" military, political and economic blows on the South Korean Government to accelerate its "pitiable demise".
South Korea's Government called the statement a "provocative act" and warned the North not to damage any South Korean assets.
North Korea didn't say what exactly it would do with the South Korean assets. Observers said it could move the remaining manufacturing equipment at Kaesong to other industrial areas or convert it to military use, and use the South Korean-owned facilities at Diamond Mountain for its own tourism project.
The South Korean assets at Kaesong include buildings, manufacturing equipment and finished products. The assets at the resort are a hotel, a spa, a golf course and a building used for reunions of Korean families separated by war.
Reacting to Kim's call for more nuclear tests, South Korea yesterday said he was being "rash" and displaying his ignorance of international opinion. "The international community is imposing strong and comprehensive sanctions and this only goes to prove why they are necessary," said Unification Ministry Spokesperson Jeong Joon Hee.
UN Secretary-General Ban Ki Moon voiced grave concern over the growing tensions, and urged North Korea to avoid any further "destabilising acts".
Kim, however, chose to highlight the need for a diversified nuclear strike force, capable of delivering warheads from the ground, air, sea and underwater. The North has conducted a number of what is says were successful tests of a submarine launched ballistic missile.
Outside experts have questioned the results of those tests, suggesting Pyongyang had gone little further than a "pop-up" test from a submerged platform.