Kim Jong-un has said his country will not use its nuclear weapons first unless its sovereignty is invaded, in a speech during a critical party congress that struck a conciliatory note in the face of international pressure over its recent nuclear test and long-range rocket launch.

Kim said he is ready to improve ties with "hostile" nations, and called for more talks with rival South Korea to reduce misunderstanding and distrust between them. He also urged the United States to stay away from inter-Korean issues, according to the official Korean Central News Agency.

"Our republic is a responsible nuclear state that, as we made clear before, will not use nuclear weapons first unless aggressive hostile forces use nuclear weapons to invade on our sovereignty," Kim said in a speech carried by the KCNA.

The North's Korean Central Television on Sunday showed Kim delivering the speech at Pyongyang's House of Culture, wearing a black dress suit, a grey tie and horn-rimmed glasses that resembled the ones worn by his late grandfather and North Korean leader Kim Il Sung.

He said that North Korea "will sincerely fulfill its duties for the non-proliferation of nuclear weapons and work to realize the denuclearization of the world."

The North is ready to improve and normalize ties with countries hostile to it if they respect its sovereignty and approach it in a friendly manner, Kim said.

Everyday life in North Korea - in pictures Show all 10 1 /10 Everyday life in North Korea - in pictures Everyday life in North Korea - in pictures 'Ordinary life' in North Korea A man cuts the hair of a young boy at an apartment building in Pyongyang. High rise apartments are a common form of accommodation for people living in the capital city AP Photo/Dita Alangkara Everyday life in North Korea - in pictures 'Ordinary life' in North Korea North Koreans pause to give way for passing vehicles as they cross a road in Pyongyang. AP Photo/Dita Alangkara Everyday life in North Korea - in pictures 'Ordinary life' in North Korea North Koreans wait for public transportation at a bus stop in Pyongyang AP Photo/Dita Alangkara Everyday life in North Korea - in pictures 'Ordinary life' in North Korea A woman and her daughter walk past a North Korean flag hung on a utility pole as part of celebrations of the Liberation Day in Pyongyang AP Photo/Dita Alangkara Everyday life in North Korea - in pictures 'Ordinary life' in North Korea People attending the conference for national reunification as they observe their 70th anniversary of Korea's liberation, marking the end of World War II, in Panmunjom AFP PHOTO / KCNA via KNS REPUBLIC OF KOREA OUT Everyday life in North Korea - in pictures 'Ordinary life' in North Korea A man looks at items at a stamp shop in Pyongyang AP Photo/Dita Alangkara Everyday life in North Korea - in pictures 'Ordinary life' in North Korea North Koreans participate in a closing event for its celebration of the 70th anniversary of Korea's independence from Japan's 1910-45 colonial rule, at the truce village inside the demilitarized zone (DMZ) that divides the two Koreas in Panmunjom, North Korea. The country changed it's timezone on the occasion. North Korea introduced 'Pyongyang time' and pushed back its clocks by half an hour on 15 August, the same as before the Japanese occupation when the standard time used by the Korean empire was eight and a half hours ahead of GMT, instead of nine hours, which is Tokyo time EPA/KCNA SOUTH KOREA OUT Everyday life in North Korea - in pictures 'Ordinary life' in North Korea People attending the conference for national reunification as they observe their 70th anniversary of Korea's liberation, marking the end of World War II, in Panmunjom AFP PHOTO / KCNA via KNS Everyday life in North Korea - in pictures 'Ordinary life' in North Korea Youths and students attending an evening gala at the Kim Il-Sung Square in Pyongyang to celebrate the 70th anniversary of Korea's liberation, marking the end of World War II AFP PHOTO / KCNA via KNS REPUBLIC OF KOREA OUT Everyday life in North Korea - in pictures 'Ordinary life' in North Korea North Korean leader Kim Jong Un visits the Kumsusan Palace of the Sun to mark the occasion of the 70th anniversary of Korea's liberation REUTERS/KCNAATTENTION EDITORS

Despite the talks about more diplomatic activity, Kim also made it clear that the North has no plans to discard its "byongjin" policy of simultaneously developing its nuclear weapons and its domestic economy.

In a speech published by the North's Rodong Sinmun newspaper, Kim described the twin policy as a strategy the party must permanently hold on to for the "maximized interest of our revolution."

Many outside analysts consider the policy unlikely to succeed because of the heavy price North Korea pays for its nuclear program in terms of international sanctions that keep its economy from growing.

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 program. He also said the country must secure more electricity through nuclear power plants, according to the state media. Jeong Joon Hee, the spokesman from the South's Unification Ministry, said that the North currently doesn't have nuclear power plants producing electricity.

Analysts have anticipated Kim would use the first Workers' Party congress in decades to propose talks with rivals to exploit what he considers to be 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 U.N. sanctions.

The North responded to the punitive measures, and also the annual U.S.-South Korean military drills in March and April, by firing a series of missiles and artillery into the sea. It also claimed advancements in developing nuclear weapons and long-range missiles, and combined them with threats of pre-emptive nuclear strikes on Washington and Seoul.

Analysts said that 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.

South Korea has taken a hard-line approach to North Korea following its nuclear test and long-range rocket launch, shutting down a jointly-run factory park in a North Korean border town that had been the last remaining symbol of cooperation between the rivals and slapping Pyongyang with its own economic sanctions.