North Korean leader Kim Jong-un vowed not to use nuclear weapons unless attacked, state-run media said Sunday.
Kim also pledged to seek to "improve and normalize" relations with "hostile" countries - generally understood to mean the United States, and South Korea - the South's Yonhap news agency reported.
"As a responsible nuclear weapons state, our republic will not use a nuclear weapon unless its sovereignty is encroached upon by any aggressive hostile forces with nukes," Kim was quoted as saying by the Korean Central News Agency.
"It will faithfully fulfill its obligation for non-proliferation and strive for global denuclearization."
His remarks came on the second day of the Workers' Pary of Korea congress, which convened for the first time in more than 35 years Friday.
The event is expected to last several days, and endorse Kim's position as supreme leader of the secretive nation.
At the congress a new five-year economic plan was also introduced, which includes improving the nation's power supply and increasing output from the agricultural and light industrial sectors, according to state media.
South Korea's Defence Ministry reacted with scepticism about Pyongyang's willingness to improve north-south relations, saying the comments where merely a part of the nation's "propaganda" and not to be taken seriously.
There is growing speculation that Pyongyang is preparing a new nuclear test soon, as satellite images have shown increased activity at an underground test site.
The North reported its first successful detonation of a nuclear device in 2006 and followed up with other tests in 2009, 2013 and in January of this year.
Kim has celebrated the tests and a long-range rocket launch earlier this year as among the country's greatest accomplishments. The United Nations has imposed its strictest sanctions on Pyongyang in response.
The party congress is the first since October 1980 and the first under Kim, who assumed power in late 2011 following the death of his father Kim Jong-il.