North Korea deploys 250 missile launchers along southern border

North Korea has deployed 250 ballistic missile launchers to its southern border, with leader Kim Jong Un describing the weapons as a "powerful treasured sword" to defend its sovereignty, state media said Monday.

The olive green mobile launchers were displayed during a special "transferring" ceremony in the capital Pyongyang on Sunday, the official Korean Central News Agency reported.

The missile launchers were an "up-to-date tactical attack weapon", Kim was quoted as saying in a speech.

It was the first time North Korea had publicised the scale of an arms transfer to its border units, Han Kwon-hee of the Korea Association of Defence Industry Studies told AFP.

Each launcher was designed to hold four missiles, Han said, adding that Pyongyang's supply of arms to Moscow might have "interfered with the North's capacity to actually produce 1,000 missiles".

Relations between the two Koreas are at one of their lowest points in years, with the North ramping up weapons testing and bombarding the South with balloons full of trash.

South Korea has responded by resuming propaganda broadcasts along the border, suspending a tension-reducing military deal and restarting live-fire drills near the border.

This year, Pyongyang declared South Korea its "principal enemy", jettisoned agencies dedicated to reunification and outreach, and threatened war over "even 0.001 mm" of territorial infringement.

Kim noted that presenting the new weapons at a time when the country was reeling from flood damage was a "manifestation of the firm will of our Party to push ahead with the bolstering of defence capabilities".

Heavy rainfall hit the nuclear-armed country's northern regions in late July, with a South Korean media report claiming up to 1,500 people could have died.

Kim has lashed out at the reports, dismissing them as a "smear campaign to bring disgrace upon us and tarnish" the North's image.

The North has said there were no casualties at all in the Sinuiju area, the region Pyongyang said had suffered the "greatest flood damage".

It claimed North Korea's Air Force rescued over 5,000 people, with around 4,200 of them saved by helicopter "within a few hours".

Kim said in his speech that the country was facing a "significant and strategic shift due to the transformation of US-led alliances into nuclear-based military blocs", a trend he said required an upgrade in war deterrence, according to a KCNA transcript.

"The strength of our armed forces... must continue to accelerate," Kim said.

Photos released by KCNA showed rows of mobile launchers positioned before a podium where Kim spoke.

Banners flown from a balloon painted with the North Korean flag read: "Let's open the golden age of defence industry development under the leadership of the great comrade Kim!"

While it was possible that Pyongyang had previously deployed ballistic missile launchers to its southern border, Han said it was the first time he had "seen such official reports detailing the scale of arms transfers" to military units in the area.

Producing such a large number of weapons and displaying them together indicates the event had been "planned months in advance", Hong Min, a senior analyst at the Korea Institute for National Unification, told AFP.

"Producing 250 mobile launchers requires massive manpower and capital, which shows that the event on Sunday was not staged on a short notice," he said.

"Pyongyang appears to have judged that staging such a performance-like event would be an effective message to the United States."

Among the event attendees was Kim's teenage daughter, Ju Ae, who Seoul's spy agency said last week was being groomed as Kim's heir.

Ju Ae, whose exact age has not been confirmed, was seen clapping behind her father in one KCNA photo.

While Kim claimed that the new weapon system was designed by himself, it is likely that they copied the raw technology from overseas, Hong added.

"It is possible that Kim directed the modification using the original technology."