North Korea's first ruling party congress for nearly 40 years formally endorsed leader Kim Jong-Un's policy of expanding the country's nuclear arsenal, as South Korea on Monday dismissed his proposals for military talks and improved ties.

The congress, which opened on Friday, has largely been seen as an elaborate coronation for the 33-year-old Kim, securing his status as supreme leader and confirming his legacy "byungjin" doctrine of twin economic and nuclear development.

On Sunday, the thousands of delegates to what is technically North Korea's top decision-making body, adopted a report submitted by Kim the day before to simultaneously push forward economic construction and "boost self-defensive nuclear force both in quality and quantity."

It also enshrined a policy of not using nuclear weapons unless its sovereignty is threatened by another nuclear power, and of working towards the eventual reunification of the divided Korean peninsula.

"But if the south Korean authorities opt for a war... we will turn out in the just war to mercilessly wipe out the anti-reunification forces," said the document published by the North's official KCNA news agency.

Reiterating the North's long-held argument that its push for a nuclear deterrent was forced by US hostility, the congress said the nuclear weapons programme would move forward "as long as the imperialists persist in their nuclear threat."

Presenting his report to the congress in a marathon three-hour speech on Saturday, Kim said Pyongyang wanted better relations with previously "hostile" nations and proposed military talks with South Korea to ease tensions on their heavily fortified border.

- 'Propaganda' -

The government in Seoul dismissed his remarks, including a vow to pursue global denuclearisation, as meaningless propaganda.

"There is absolutely no sincerity in talking about the necessity of military talks ... while calling oneself a nuclear weapons state and launching nuclear and missile provocations," defence ministry spokesman Moon Sang-Gyun said.

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Moon said the party congress had only served to reaffirm North Korea's intention to develop its nuclear arsenal, and added that Seoul would continue to counter those ambitions with sanctions and pressure.

The South Korean Unification Ministry was equally dismissive, describing Kim's remarks on improving North-South ties as a "propaganda act with no sincerity."

North Korea has carried out four nuclear tests -- two of them under Kim's leadership.

The North said its most recent test in January was of a powerful hydrogen bomb, although experts questioned the claim given the relatively low yield.

There is growing concern that Pyongyang may be on the verge of conducting a fifth test, with satellite imagery showing activity at the North's Punggye-ri nuclear test site.

- Securing power -

Kim was not even born when the last party congress was held in 1980 to crown his father, Kim Jong-Il, as the heir apparent to founding leader Kim Il-Sung.

When his own turn came, following the death of Kim Jong-Il in December 2011, the new young leader quickly set about cementing his power base and securing his legitimacy as the inheritor of Kim family's ruling dynasty.

One of his earliest moves was to adjust his father's "songun", or military first policy, to the "byungjin" policy of economic-nuclear development.

The nuclear half of that strategy had dominated the run-up to the party congress, starting with a fourth nuclear test in January that was followed by a long-range rocket launch and a flurry of other missile and weapons tests.

Some observers had predicted that the congress might switch the focus to the economic side of the equation, and Kim did unveil a five-year economic plan -- the first of its kind for decades.

But his report to the congress offered few details of the plan's policies or targets beyond general ambitions to boost production across all economic sectors, with a particular focus on energy production.