North Korea ramped up its nuclear threats on Wednesday, boasting of its ability to deliver miniaturized warheads on high-precision long range rockets -- a claim denied by the United States.

At the same time, the North denounced "reckless" remarks made in Seoul this week by U.S. Secretary of State John Kerry, and canceled an invitation to U.N. Secretary General Ban Ki-Moon to visit a joint industrial zone on its side of the border with South Korea.

North Korea on Wednesday cancelled an invitation to U.N. chief Ban Ki-moon to visit an industrial zone inside the isolated state, in an abrupt move that Ban deplored as "deeply regrettable".

The decision comes after Ban this week urged North Korea to avoid any actions that might escalate military tensions, after it successfully test-fired a submarine-launched ballistic missile (SLBM). 

U.N. Secretary General Ban Ki-moon on Tuesday urged North Korea to avoid any actions that might escalate military tensions as South Korea's president spoke again of a "reign of terror" in the North's capital, Pyongyang.

Speaking at an Asian Leadership Conference in South Korea's capital, Seoul, Ban said the divided Korean peninsula remained one of the world's most dangerous flashpoints.

South Korea's military said it was holding live-fire drills on Tuesday off its country's east coast involving anti-ship missiles and jet fighters after North Korea conducted a submarine-launched ballistic missile test condemned by Seoul and Washington.

The joint naval and air force exercise involves some 20 warships as well as an unspecified number of jet fighters, Seoul's defence ministry deputy spokesman Na Seung-Yong told reporters.

U.S. Secretary of State John Kerry warned North Korea Monday of possible fresh sanctions as he slammed Kim Jong-Un's "egregious" leadership with its reliance on reckless provocation and "grotesque" executions.

Speaking in the South Korean capital Seoul, Kerry said the recent test of a new submarine-launched ballistic missile (SLBM) showed that North Korea had no genuine interest in engaging with the international community.

North Korea carried out a live-fire drill near the disputed sea border with South Korea Thursday for the second consecutive day, snubbing calls from Seoul not to escalate cross-border tensions.

The drill began at 7:10 pm (1010 GMT) near a South Korean front-line island close to the maritime border in the Yellow Sea, the South's Defense Ministry said.

North Korea's recent test of a submarine-launched ballistic missile was probably genuine but almost certainly exaggerated, and did not present the imminent threat promoted by Pyongyang, weapons experts and analysts said Thursday.

Reaffirming suspicions that last Friday's exercise was an "ejection" test rather than a full-scale test, the experts said the North was still likely in the early stages of developing a credible SLBM capability.

North Korea launched live-fire drills near the disputed sea border with South Korea late Wednesday, the Yonhap news agency reported, amid high tensions over Pyongyang's recent submarine-launched ballistic missile test.

The reported firing drills -- some shot from a war ship -- came just hours after Pyongyang informed South Korea of its plans to conduct the exercise.

North Korean leader Kim Jong-Un had his defense minister executed with anti-aircraft fire for insubordination and dozing off during a formal military rally, South Korean intelligence said Wednesday, hinting at possible instability in the Pyongyang leadership.

If confirmed, it marks another demonstration of Kim's ruthlessness in dealing with even the most senior officials suspected of disloyalty, following the execution of his uncle and one-time political mentor Jang Song-Thaek in 2013.

North Korea's claim that it launched a ballistic missile from a submarine was mostly likely a bluff and the images it released appear photoshopped to overstate the regime's military power, U.S. experts said Tuesday.

Pyongyang state media announced triumphantly on Saturday that a new submarine-launched ballistic missile had been tested successfully, under the personal supervision of the country's leader Kim Jong-Un.