North Korean dictator Kim Jong Un, after watching a ballistic missile test launch, ordered the country to improve its nuclear attack capability by conducting more tests, the official KCNA news agency reported.
The report 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 500 km 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 the sanctions. 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 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.
In China, North Korea's most important economic and diplomatic backer, the top newspaper, the People's Daily, urged all sides to be "patient and brave", show goodwill and resume the talks process.
South Korea said it did not believe that North Korea had successfully miniaturised a nuclear warhead or deployed a functioning intercontinental ballistic missile. The US Defence Department said this week it had seen no evidence that North Korea had succeeded in miniaturising a warhead.
However, Admiral Bill Gortney, the officer responsible for defending US air space, told a US Senate panel on Thursday it was "prudent" for him to assume North Korea could both miniaturise a warhead and put it on an ICBM that could target the United States.
"Intel community gives it a very low probability of success, but I do not believe the American people want (me) to base my readiness assessment on a low probability," he said. North Korea has issued nearly daily reports in recent days of Kim's instructions to fight South Korea and the United States as the two allies began large-scale military drills.
North Korea called the annual drills "nuclear war moves" and threatened to respond with an all-out offensive. Kim last week ordered his country to be ready to use nuclear weapons in the face of what he sees as growing threats from enemies.
The United States and South Korea remain technically at war with North Korea because the 1950-53 Korean War ended in a truce instead of a peace agreement.