PYONGYANG (Reuters) - North Korea said it would strengthen its defensive nuclear weapons capability, the official KCNA news agency reported on Monday, a decision adopted in defiance of U.N. resolutions at a rare congress of its ruling Workers’ Party.

The congress, which ended on Monday after four days, was the first in 36 years, and secretive 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.

Young leader Kim Jong Un, who assumed power in 2011 after his father’s sudden death, took on the new title of party chairman on Monday, media reported.

The promotion - his previous party title was first secretary - had been predicted by analysts who had expected Kim would use the congress to consolidate his power.

North Korea has come under tightening international pressure over its nuclear weapons program, including tougher U.N. 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 formalizes 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 U.N. resolutions, North Korea has pressed ahead with its nuclear and missile development, and said it had succeeded in miniaturizing a nuclear warhead and launching a submarine-based ballistic missile.

Late on Monday, North Korean state TV showed Kim declaring the congress closed.

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RIVAL KOREAS

South Korea condemned the North’s claim to be a nuclear weapons state, saying it would continue to exert pressure on Pyongyang until it abandons its nuclear ambitions.

North Korea is believed by western experts to have about 40 kg of plutonium, enough to build eight to 12 nuclear weapons.

On the weekend, Kim took a conciliatory position on ties with the South, saying military talks were needed to discuss ways to ease tension.

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South Korea rejected the proposal as meaningless.

“We have not given up on dialogue,” South Korean Unification Ministry spokesman Cheong Joon-hee told a briefing. “But it is only when the North shows sincerity about decentralization that genuine dialogue is possible.”

The unusually large group of 128 foreign media members in Pyongyang for the congress had not been given any access to the proceedings until Monday afternoon, when a group of about 30 of them were let in to the April 25 House of Culture for several minutes after nearly three hours of security checks.

There, Kim entered and was received by a wildly cheering audience of delegates, according to reporters who got in.

The expulsion of BBC journalist Rupert Wingfield-Hayes grabbed headlines in foreign media on Monday. He had been in the country ahead of the congress to cover the visit of a group of Nobel laureates.

Earlier on Monday, visiting media were taken to a textile factory named after Kim Jong Suk, the wife of state founder Kim Il Sung and the grandmother of the current leader.

They have been taken to a string of show-case sites including a maternity hospital, electric cable factory and children’s center.