SEOUL- North Korea said Monday it had a secret strike force to counter what it called South Korean plots to topple the regime in Pyongyang.

The communist North, which often claims that Seoul's conservative government is plotting against it, said such attempts have "recently gone beyond the danger line."

A joint statement from the Ministry of People's Security and the Ministry of State Security cited Seoul's support for US demands that Pyongyang scrap its nuclear weapons before any broader settlement of differences.

The statement, carried by the official news agency, also criticised efforts by the South's military to defend the disputed Yellow Sea border and "reckless" operations to destabilise the North.

"We have world-level ultra-modern striking force and means for protecting security which have neither yet been mentioned nor opened to the public in total," it said without elaborating.

The North has tested two plutonium-based atomic weapons. Last September it said it had reached the final stage of an experimental uranium enrichment programme, an alternative way of making nuclear bombs.

Monday's statement complained about "the daily escalating" scattering of propaganda leaflets, which it said were now penetrating deep into the country from border areas.

Defectors and other anti-Pyongyang groups often launch balloons carrying such leaflets across the border. The Seoul government says it has asked them to stop but has no law to ban them.

The leaflets often call for the overthrow of leader Kim Jong-Il.

Pyongyang's statement claimed that Seoul's ministries along with conservative forces and "human scum" are involved in destabilisation efforts.

It accused South Korean authorities of putting into practice a military contingency plan to handle regime collapse north of the border.

The North reacted furiously last month to media reports that Seoul has such a plan, threatening a "holy war" against its neighbour.

There have been reports of unrest in the North sparked by a shock currency revaluation said to have worsened food shortages and fuelled inflation.

South Korea's spy chief has been quoted as saying the revaluation sparked riots but that the government appeared now to have things under control.

The spy agency said last week the North is relaxing restrictions on private markets because of mounting anger at the chaotic currency change.

Kim's health is also a source of uncertainty after he suffered a stroke in August 2008.