According to North Korea, the February 26 and March 4 tests were focused on developing a reconnaissance satellite, but the US official said rigorous analysis concluded they were actually experimental precursors to a likely full-range ICBM launch.

Any such launch would mark the end of a self-imposed moratorium Pyongyang has had in place since 2017 and send military tensions soaring on the Korean peninsula and beyond.

The North has carried out three ICBM tests; the last in November 2017 of a Hwasong-15 -- deemed powerful enough to reach Washington and the rest of the continental United States.

The US official said the recent two tests "involved a relatively new intercontinental ballistic missile system" that Pyongyang had first showcased at a military parade in October 2020.

"This is a serious escalation," the official said, adding that while neither launch displayed ICBM range or capability, they were clearly intended "to test elements of this new system before (North Korea) conducts a launch in full range."

When the full test is carried out, North Korea will likely seek to disguise it as a "space launch," the official said.

Prior to its ICBM tests in 2017, the North had carried out a series of powerful rocket launches that it insisted were part of a wider civilian space programme.

North Korea is already under biting international sanctions over its missile and nuclear weapons programme, and the official said the US Treasury would announce fresh measures on Friday to help prevent Pyongyang accessing "foreign items and technology" to advance that programme.

Such measures underline that the North's "unlawful and destabilising activities have consequences" and that diplomatic negotiations are the only viable path forward for Pyongyang, the official said.

- Nine weapons tests so far -

When the new ICBM was unveiled at the 2020 parade, military analysts said it appeared to be the largest road-mobile, liquid-fuelled missile anywhere in the world -- and likely designed to carry multiple warheads in independent re-entry vehicles (MIRVs).

North Korea watchers regularly caution that the devices Pyongyang puts on show at its parades may be mock-ups or models, and there is no proof they work until they are tested.

Pyongyang has been abiding by its moratorium on testing ICBMs and nuclear weapons since leader Kim Jong Un embarked on a flurry of high-profile diplomatic engagement with then US president Donald Trump in 2017.

Talks later collapsed and diplomacy has languished ever since, despite efforts by the administration of US President Joe Biden to offer fresh negotiations.

The North started hinting in January that it might lift the moratorium, and it has conducted nine weapons tests this year, including of banned hypersonic and medium range ballistic missiles.

Earlier Thursday, North Korean state media reported that Kim had visited the space center in Pyongyang and praised scientists there for their work on the "reconaissannce missile."

A fresh ICBM launch would be an early challenge for South Korea's new president-elect, Yoon Suk-yeol, who has vowed to take a hard line with the North's provocations.

Yoon has not ruled out the possibility of dialogue with Pyongyang, but analysts say his hawkish position puts him on a completely different footing and significantly reduces the prospect of substantive engagement.