North Korea leader Kim Jong Un becomes chairman of ruling Workers' Party: NHK

North Korean leader Kim Jong Un, who has been first secretary of the ruling party, became chairman of the Workers' Party of Korea at a rare party congress held in Pyongyang, Japan's public broadcaster NHK reported on Monday.

The first congress of the Workers' Party in 36 years, which opened on Friday, was scheduled to promote Kim to the "top post" of the party, according to state media.

The secretive North Korea also said it will strengthen self-defensive nuclear weapons capability, its KCNA news agency reported on Monday, a decision adopted in defiance of UN resolutions at a rare congress of its ruling Workers' Party.

The congress is the first in 36 years and North Korea granted visas to scores of foreign journalists to coincide with the gathering.

Their movements have been closely monitored and one BBC journalist, not reporting directly on the congress, was expelled along with two colleagues, after a top official said he had "distorted facts and realities" in his coverage.

North Korea has come under tightening international pressure over its nuclear weapons programme, including tougher UN sanctions adopted in March backed by lone major ally China, following its most recent nuclear test in January.

The congress's decision on strengthening the capability of its nuclear weapons formalises North Korea's position. It had already declared itself "a responsible nuclear weapons state" and disavowed the use of nuclear weapons unless its sovereignty is first infringed by others with nuclear arms.

"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," KCNA said, citing the congress.

The two Koreas remain in a technical state of war since their 1950-53 conflict ended in a truce, not a peace treaty. North Korea regularly threatens the South and its major ally, the United States, which it accuses of planning a nuclear attack.

Since the latest round of UN resolutions, North Korea has pressed ahead with its nuclear and missile development, and said it had succeeded in miniaturising a nuclear warhead and launching a submarine-based ballistic missile.