Chinese state-run media on Monday played down North Korean leader Kim Jong-Un's pledge not to use nuclear weapons unless his country's sovereignty is threatened, saying that his pursuit of atomic arms remained dangerous.

Beijing is Pyongyang's main diplomatic protector and source of trade and aid, but relations between them have become increasingly strained by the North's nuclear ambitions, and Kim has yet to visit his neighbour.

The North's first ruling party congress in nearly 40 years formally endorsed Kim's policy of expanding the country's nuclear arsenal, after he said it would not use the weapons unless attacked, and would work for global denuclearisation.

But the international community and the United Nations have long demanded an end to the North's nuclear and missile programmes.

Kim's declaration "was made from the perspective that North Korea is now a nuclear state", China's Global Times newspaper, which is close to the ruling Communist party, said in an editorial.

As such, it said, its "attitude has not changed, and neither has its biggest contradiction with the outside world been resolved".

"Major countries will not change their stance to recognise North Korea as a nuclear state," it added. "As long as Pyongyang resists giving up its nuclear weapons, normalising relations with the outside world will be highly unlikely."

China's foreign ministry said that Beijing's position on the nuclear question "remains unchanged" following the weekend's events.

"We maintain that all resolutions of the UN Security Council related to the issue should be applied by all the parties," spokesman Lu Kang told reporters at a regular briefing.

There were no Chinese representatives at the Workers' Party gathering, the Global Times reported last week, although a large delegation attended the previous congress in 1980 headed by Li Xiannian, later China's official head of state.

Beijing has been reluctant to take measures against the North, fearing that a crisis could send floods of refugees into its territory. It also views as anathema the prospect of US troops on its border in a reunified Korea.

The North's nuclear programme had been a factor in the US and South Korea "constantly upgrading their military preparation for strikes against Pyongyang", the newspaper said.

"The crazy logic of contemporary international politics has become a game of who will blink first," it added.