North Korean leader Kim Jong Un took on the new title of chairman of the ruling Workers' Party on the fourth day of a rare congress widely seen as a platform for him to consolidate power.
The newly created post - a promotion for Mr Kim, 33, who was previously first secretary - was announced yesterday and witnessed by thousands of delegates and more than 100 foreign journalists.
At the congress, the first since 1980, the party also adopted a decision to pursue nuclear and economic development together.
Pyongyang on Sunday said it would boost its nuclear self-defence capability in both quality and quantity, reported the North's Korean Central News Agency yesterday.
"We will consistently take hold of the strategic line of simultaneously pushing forward the economic construction and the building of nuclear force... as long as the imperialists persist in their nuclear threat and arbitrary practices," the agency reported, citing the congress.
Since assuming the leadership in 2011, after the death of his father, Mr Kim Jong Il, the younger Mr Kim has pursued a dual-track policy, known as "byeongjin", to simultaneously engage in nuclear and economic development.
The reclusive state has conducted two nuclear tests under his charge, most recently in January.
Mr Kim declared in a speech at the congress televised on Sunday that North Korea is a "responsible nuclear state" that will not use its nuclear arms first unless it is under attack. He said North Korea would "faithfully fulfil its obligation for non-proliferation and strive for global denuclearisation", a statement which experts interpret as the country's demand to be recognised as a nuclear power.
South Korean officials have dismissed Mr Kim's claims as insincere and mere propaganda.
Its Defence Ministry yesterday said North Korea will never be recognised as a nuclear power by South Korea and the international community, and that Seoul will continue to exert pressure and uphold sanctions to force Pyongyang to give up its nuclear ambitions.
In Washington, South Korean and United States officials were set yesterday to discuss details of an operational plan to strengthen South Korea's missile detection and defence systems against any potential attack from the North.
Meanwhile, China's state-run media Global Times said in an editorial yesterday that North Korea's attitude has not changed, and normalising ties with the outside world will be highly unlikely "as long as Pyongyang resists giving up its nuclear weapons".
North Korea wrapped up the congress yesterday, after the election of a new party leadership.
Observers said the regime will likely revise rules to reflect its nuclear state claim.
Dr Lee Seong Hyun, a research fellow at the Sejong Institute think-tank, called Mr Kim's nuclear declarations a "deceptively clever word game". He said it creates the false appearance that North Korea is willing to give up nuclear weapons when, in fact, it is not.