North Korean leader Kim Jong Un watched a ballistic missile launch test and ordered the country to improve its nuclear attack capability by conducting more tests, the official KCNA news agency reports.
The report on Friday (local time) did not say when the test took place but it was probably referring to North Korea's launch of two short-range missiles on Thursday that flew 500km and splashed into the sea.
"Dear comrade Kim Jong Un said work ... must be strengthened to improve nuclear attack capability and issued combat tasks to continue nuclear explosion tests to assess the power of newly developed nuclear warheads and tests to improve nuclear attack capability," KCNA said.
The North Korean leader was quoted in state media this week as saying his country had miniaturised nuclear warheads to mount on ballistic missiles.
Tensions have risen sharply on the Korean peninsula after the North conducted its fourth nuclear test in January and fired a long-range rocket last month, spurring the UN Security Council to adopt a new sanctions resolution.
Conducting more nuclear tests would be in clear violation of UN sanctions, which also ban ballistic missile tests, although Pyongyang has rejected them. North Korea has a large stockpile of short-range missiles and is developing long-range and intercontinental ballistic missiles (ICBMs).
South Korea's Unification Ministry spokesman Jeong Joon-hee said, "It's simply rash and thoughtless behaviour by someone who has no idea how the world works," when asked about Mr Kim's comments.
UN Secretary-General Ban Ki-moon urged Pyongyang to "cease destabilising acts," adding that he remained "gravely concerned" by the situation.
North Korea has recently stepped up its cyber attack efforts against South Korea and succeeded in hacking the mobile telephones of 40 of its national security officials, said members of parliament who received a closed door briefing by the country's spy agency.
South Korea has raised its alert against the threat of the North's cyber attacks and this week said it had intercepted attempts to attack its railway system.