SEOUL (BLOOMBERG) - North Korea looks set to detonate its first nuclear bomb in more than four years, as America's sanctions disputes with Russia and China make further United Nations (UN) penalties against the country unlikely.

Workers have been observed digging a new passageway at the Punggye-ri site where North Korea conducted all six of its previous nuclear tests, South Korean media including the DongA Ilbo newspaper reported.

A test could come as soon as next month, when Mr Kim Jong Un is preparing to mark the 110th anniversary of the birth of his grandfather, state founder Kim Il Sung, the paper said, citing a security official it didn't identify.

The reports came as Mr Kim delivered a fresh warning to the United States that he planned to develop more "powerful striking capabilities", the official Korean Central News Agency reported Monday (March 28).

The weapons would make North Korea more secure and "control all threats and blackmail by the imperialists", it reported him as saying.

While Mr Kim has been signalling plans to resume major weapons tests for more than two years, the US campaign to punish Russia over its invasion of Ukraine has reduced the risk of getting sanctions for such provocations.

Any additional measures from the UN Security Council would require support from Russia, as well as China, which has led the criticism of Washington's efforts to squeeze Moscow economically.

Their reluctance was clear during a Security Council meeting Friday to discuss North Korea's first launch of an intercontinental ballistic missile since November 2017.

Russia's representative, Ms Anna Evstigneeva, rejected US calls for what she described as "turning the sanctions screw" against North Korea and advocated for a resolution drafted with China that would prioritise negotiations.

"North Korea almost certainly views the rifts between the US and Russia, and the US and China, as a golden opportunity to conduct longer-range missile - and probably even nuclear - tests," said non-resident fellow Rachel Minyoung Lee, with the 38 North Programme at the Stimson Centre.

The response to the ICBM test shows how much the geopolitical landscape has shifted since 2017, when former US President Donald Trump threatened to unleash "fire and fury" against North Korea and secured China and Russia's support for tough UN sanctions against the regime.

Mr Trump subsequently launched a trade war against China and opened direct talks with Mr Kim, prompting President Xi Jinping in Beijing to repair ties with Pyongyang, as well.

The war in Ukraine has complicated things further, as President Joe Biden's administration frames his showdown with Russia's Vladimir Putin as part of a global battle between democracy and autocracy.

At the same time, the conflict has underscored the value of nuclear weapons in deterring direct military action by the US and its allies.

North Korea was one of five UN member states last month to vote against a resolution condemning the invasion. Pyongyang issued a statement defending the attack, saying the blame "totally lies in the hegemonic policy of the US and the West which indulge themselves in high-handedness and arbitrariness towards other countries".

Although North Korea still languishes under a range of UN sanctions, it has continued to roll out an array of new missiles that would require smaller, more advanced warheads to pose a credible deterrent to the US.

Such weapons could increase Kim's leverage if he decides to return to nuclear disarmament talks and ease the sanctions choking the economy.

In the closing days of Mr Trump's term, Mr Kim laid out a five-year road map for his nuclear programme that included making warheads smaller and lighter, as well as more powerful.

Weapons mentioned in that speech, such as hypersonic missiles that are designed to evade US-operated missile shields and hit American bases in South Korea and Japan, have been among the those tested in recent months.

The recent work at the Punggye-ri site appears designed to reverse Mr Kim's efforts to dismantle the site before his first landmark summit with Mr Trump in June 2018.

That demolition was intended to demonstrate North Korea's commitment to a self-imposed testing freeze that Mr Kim later declared null-and-void after talks with Trump fell apart.

Observers will be watching where in the site's various tunnels any bomb is detonated for clues about Mr Kim's aims, said senior research fellow Lee Choon-geun at South Korea's Science and Technology Policy Institute.

South Korea's military has detected signs of work at tunnel No 3, the DongA newspaper said.

"If North Korea conducts its next nuclear test in the third tunnel, then it is testing its nuclear-warhead-miniaturization technology for tactical nuclear weapons," Lee Choon-geun said.

A fourth tunnel was believed to be reserved for testing thermonuclear devices, he said.

Top military officials from Japan, South Korea and the US are set to meet this week in Hawaii to discuss issues including the recent ICBM launch, Yonhap News Agency reported Tuesday, citing an unnamed source.

On Friday, North Korea claimed it fired off a Hwasong-17, which weapons experts believe is the largest road-mobile missile of its kind and designed to overwhelm US defences with multiple warheads.

That launch and the slick, highly produced video North Korea released to promote it shows that Mr Kim is using such tests to build national unity.

But Mr Kim might have launched another, less-advanced ICBM after an earlier attempt to fire the bigger missile ended in failure, the Yonhap News Agency reported, citing a senior official in Seoul.

The video was then potentially edited to make it look like the more advanced missile was launched successfully, according to an analysis by NK News.

"Kim regime is using these tests to unify the domestic public, give the people a sense of pride, and even justify their economic difficulties in the name of strengthening the country's military capabilities," Rachel Minyoung Lee said.