North Korea said it would further strengthen self-defensive nuclear weapons capability "in quality and quantity" in a decision adopted at a rare Workers' Party congress, its KCNA news agency reported on Monday.

The decision was adopted after a review of the work of the party's Central Committee on day three of the congress on Sunday, KCNA said.

The congress is the first to be held in 36 years amid anticipation by the South Korean government and experts that leader Kim Jong Un would use it to further consolidate power. Kim became leader in 2011 after his father's sudden death.

The congress also adopted in the decision to disavow the use of nuclear weapons unless its sovereignty is first infringed by others with nuclear arms, as Kim laid out in an address on Saturday.

"We will consistently take hold on the strategic line of simultaneously pushing forward the economic construction and the building of nuclear force and boost self-defensive nuclear force both in quality and quantity as long as the imperialists persist in their nuclear threat and arbitrary practices," it said.

North Korea came under the latest U.N. sanctions in March after its fourth nuclear test in January and a long-range rocket launch the next month, but it has defied international pressure with more activities under its nuclear and missile programs.