North Korean state media on Friday released images of leader Kim Jong Un supervising tests of a multiple rocket launcher system, a day after Seoul accused Pyongyang of firing a volley of short-range ballistic missiles.
The photos showed Kim, in a brown leather jacket, smiling with uniformed generals as he supervised the simultaneous launch of what appeared to be 18 projectiles.
The test involved "super-large multiple rocket sub-units", according to a report by the official Korean Central News Agency.
Analysts have suggested the nuclear-armed North could be testing and ramping up production of artillery and cruise missiles before sending them to Russia for use in Ukraine, something the Pentagon said it had confirmed in a report released this week.
Images from the drill showed the 600mm multiple launch rocket system (MLRS), which North Korea has said can be fitted with nuclear warheads.
The exercises were meant to "serve as an occasion in clearly showing what consequences our rivals will face if they provoke us," the KCNA report said.
The drills showed that the North "will not hesitate to carry out a preemptive attack by invoking the right to self-defence at any time," it added.
KCNA said the rockets fired had "accurately hit an island target 365 km (226 miles) away".
On Thursday, South Korea's military said it had detected the launch of around 10 short-range ballistic missiles.
Seoul's military also put the range of those missiles at about 350 kilometres (217 miles), while calling the launch a "provocation".
US State Department spokesman Matthew Miller condemned the firing of the ballistic missiles -- a violation of UN sanctions -- as "reckless behavior which poses a grave threat to the Korean Peninsula".
On Monday, North Korea attempted to put a second spy satellite into orbit, but it ended in a mid-air explosion.
The attempt came just hours after Seoul, Beijing and Tokyo held a rare trilateral summit, where they called for Pyongyang to give up its nukes.
A day later, North Korea sent hundreds of trash-filled balloons across the border, in what it described as retaliation for balloons full of anti-Kim propaganda sent northwards by activists in the South.
Analysts have said North Korea's rocket launcher systems are capable of hitting Seoul, which is only some 30 miles (48 kilometers) from the Demilitarized Zone that separates the two countries.
Yang Moo-jin, president of Seoul's University of North Korean Studies, said Pyongyang's latest actions, including the trash-filled balloons, were part of an attempt to divert attention away from the satellite launch failure.
North Korea was also "trying to convey a message that the military initiative on the Korean Peninsula belongs to Pyongyang, not South Korea or the United States," he said.
In a separate KCNA report Friday, North Korea accused Washington of deploying its RC-135U reconnaissance aircraft from Japan to the Korean peninsula earlier this week.
"Other spying aircraft of the US" and South Korea's air force, "including U-2S and RQ-4B, (engaged) in round-the-clock monitoring and spying on the DPRK, seriously violating its sovereignty and security," it said, using North Korea's official name.
"The US and other hostile forces are bound to meet unforeseen disaster for their bluffing and reckless espionage," the report added.
A new report by the Pentagon's Defense Intelligence Agency, meanwhile, said an analysis of debris found in Ukraine's Kharkiv region in January confirmed Russia was using North Korean ballistic missiles in its invasion.
The North recently said the country would equip its military with a new 240mm multiple rocket launcher starting this year, adding a "significant change" for the army's artillery combat capabilities was under way.
