Pyongyang has not tested inter-continental ballistic missiles or nukes since 2017, putting launches on hold as leader Kim Jong Un embarked on a blitz of high-level diplomacy, meeting then-US president Donald Trump three times before talks collapsed two years later.

Since then, the nuclear-armed North has rebuffed US offers of talks while restarting some testing, including of hypersonic missiles, as Kim pursues his avowed goal of further strengthening his military.

When Washington imposed fresh sanctions last week, Pyongyang said it was a "provocation" and ramped up conventional weapons tests, vowing a "stronger and certain" response to efforts to rein it in.

"The hostile policy and military threat by the US have reached a danger line that can not be overlooked any more," a report on a meeting of the country's Politburo in state media KCNA said Thursday.

The North's top officials "unanimously recognized that we should make more thorough preparation for a long-term confrontation with the US imperialists," KCNA reported.

This includes examining restarting all temporarily-suspended activities, the report added.

The potential resumption of tests of nuclear weapons and long-range ballistic missiles capable of hitting the continental United States come at a delicate time in the region, with Kim's sole major ally China set to host the Winter Olympics next month and South Korea gearing up for a presidential election in March.

- '2017 again' -

North Korea bided its time during US President Joe Biden's first year in office, but with no offer for top-level talks, they've moved on, said Hong Min of the Korea Institute for National Unification in Seoul.

"It's practically 2017 again," he said, referring to a year in which Pyongyang tested nukes and ICBMs as "little rocket man" Kim Jong Un exchanged barbs with "dotard" Trump.

"With the North's announcement, it seems inevitable they'll conduct ICBM launches down the road," he said.

Ankit Panda of the Carnegie Endowment for International Peace agreed that while nuclear testing was unlikely, "long-range missile testing is back on the table."

Kim Jong Un is "reiterating a message he'd delivered back in late-2019: that US actions give him no reason to adhere to his self-imposed moratorium."

Kim had put new long-range missile launches on his military modernisation agenda last January but had always tied a return to such tests to US actions, Panda said.

"The latest round of sanctions, unfortunately, appear to have precipitated this step," he added.

The wording of the latest KCNA missive, however, indicates that "Pyongyang may be leaving some space for flexibility, depending on how the Biden administration responds," said Rachel Minyoung Lee of the Stimson Center.

Earlier this week the United States called on the country to "cease its unlawful and destabilizing activities" after it said it would seek new UN sanctions on North Korea.

But China's special representative on Korean peninsula affairs poured cold water on the idea of a security council meeting to discuss fresh curbs on the North's already-struggling economy.

"The #SecurityCouncil has no plan to discuss the so-called draft resolution concerning sanctions on the #DPRK," Liu Xiaoming wrote on Twitter.

Even as it flexes its military muscles, North Korea, reeling economically from a self-imposed coronavirus blockade, has quietly restarted cross-border trade with China.

A freight train from North Korea arrived at the Chinese border city of Dandong for the first time since early 2020 last weekend.