North Korean leader Kim Jong Un has said his country will not use its nuclear weapons first unless its sovereignty is invaded.

He is also ready to improve ties with "hostile" nations in a diplomatic overture in the face of international pressure over its recent nuclear test and long-range rocket launch, according to the reclusive communist state's official Korean Central News Agency.

The news agency said Kim, during a critical ruling party congress, called for more talks with rival South Korea to reduce misunderstanding and distrust between them and told the US to stay away from inter-Korean issues.

"(Our republic) will sincerely fulfil its duties for the non-proliferation of nuclear weapons and work to realise the de-nuclearisation of the world," Kim said.

Analysts have anticipated Kim to use the first Workers' Party congress in decades to propose talks with rivals to exploit what he considers as increased leverage as a nuclear power.

North Korea carried out its fourth nuclear test in January and followed with a satellite launch in February that was seen by outside governments as a banned test for long-range missile technology, earning worldwide condemnation and tougher United Nations sanctions.

South Korea's foreign ministry said ahead of the North's ruling party congress in Pyongyang that the priority of any future talks with the North would be its de-nuclearisation.

Analysts said the North's belligerent stance might have been intended at rallying North Korean people around Kim ahead of the congress and also promote military accomplishments to the domestic audience to make up for the lack of tangible economic achievements to present at the party meeting.

At the congress, Kim also announced a five-year plan starting this year to develop the North's dismal economy and identified improving the country's power supply and increasing its agricultural and light-manufacturing production as the critical parts of the programme.