North Korea vowed "merciless" reprisals on Tuesday after a South Korean military official suggested the isolated Stalinist nation should simply disappear.

Speaking to reporters in Seoul on Monday, Defense Ministry spokesman Kim Min-Seok had argued that North Korea barely deserved to be regarded as a proper country and that it would be best if it "vanished as soon as possible."

A state-run North Korean website on Tuesday warned a South Korean official that he would pay a "steep" price for suggesting the isolated Stalinist nation should simply disappear.

Speaking to reporters in Seoul on Monday, Defence Ministry spokesman Kim Min-Seok had argued that North Korea barely deserved to be regarded as a proper country and that it would be best if it "vanished as soon as possible."

North Korea on Monday denounced South Korea's claim that Pyongyang sent a trio of spy drones across the border as a provocative "charade" organized by its "prostitute" president, Park Geun-Hye.

South Korea's Defense Ministry said last week that an investigation into three crashed drones had provided "smoking gun" proof that they were all flown from the North.

North Korea accused South Korea Sunday of fabricating a story about crashed spy drones in order to divert attention from its ferry disaster.

The drones were recovered in three different locations in the South between March 24 and April 6.

The United States has condemned "ugly and disrespectful" racist comments directed at Barack Obama by North Korea's official KCNA news agency comparing U.S. President Barack Obama to a black "monkey" in a zoo.

North Korean propaganda is known for vitriolic personal attacks on foreign leaders, but the KCNA dispatch published last Friday -- not long after Obama's visit to South Korea -- stood out for its use of highly inflammatory and abusive racist language.

South Korean activists on Saturday launched around 200,000 anti-North leaflets across the border, a move that will anger Pyongyang which insists Seoul should forcibly prevent such events.

Around 30 campaigners led by a former North Korean defector Park Sang-Hak sent bundles of the leaflets, attached to large helium balloons, from the border city of Paju in a bid to urge the North to end human rights abuses.

North Korea signaled a key leadership change with the announcement Friday that the man seen as supreme leader Kim Jong-Un's number two had been replaced as political chief of the military.

In a report on May Day celebrations in Pyongyang, the North's official KCNA news agency named Hwang Pyong-So as the director of the Korean People's Army (KPA) General Political Bureau, not Choe Ryong-Hae, who previously held the position.

New satellite imagery indicates North Korea has been testing the engine for an inter-continental ballistic missile, a U.S. think-tank said Friday, amid concerns the North is also preparing a nuclear test.

The U.S.-Korea Institute at Johns Hopkins University said images of the North's main rocket launch site suggested one "and maybe more" tests of what is probably the first stage of a KN-08 road-mobile ICBM.

New satellite imagery confirms continued activity at North Korea's main nuclear test site that is consistent with preparations for an atomic detonation, analysts said Wednesday.

Images taken on Tuesday show particular activity near the entrances to the two completed tunnels in the South Portal area of the Punggye-ri test site, the Institute for Science and International Security (ISIS) said.

North Korea said Tuesday it would strengthen its nuclear deterrent following President Barack Obama's "dangerous" Asian tour, and would not rule out another atomic test.

There are concerns the North is preparing to conduct its fourth atomic detonation, with recent satellite images showing stepped-up activity at its main nuclear test site.