"We have seen the reports of North Korea's claims to have developed new engine technology for its ICBMs," read a statement from State Department Deputy Spokesman Mark Toner.
"We call on North Korea to refrain from actions and rhetoric that further destabilize the region and focus instead on taking concrete steps toward fulfilling its commitments and international obligations," Toner said.
North Korea said its breakthrough would "guarantee" an eventual nuclear strike on the US mainland.
(Reopens FES25)
In a statement, the US Geological Survey said a magnitude 5.3 earthquake was detected near North Korea's nuclear test site.
According to CNN, the US Air Force is expected to start flying its "radiation sniffer" jet off the Korean Peninsula in the coming hours to take air samples to see if it can determine a nuclear event occurred in North Korea.
"The Air Force WC-135 jet, dubbed the "Constant Phoenix," will look for distinctive elements a nuclear test of any type would emit into the air. The collected samples can be analysed to determine exactly what occurred," it said.
"The event seems to have been slightly larger than the one our system recorded on 6 January this year and the location is very similar to that event. Our initial location estimate shows that the event took place in the area of the DPRK's nuclear test site.
"If confirmed as a nuclear test, this act constitutes yet another breach of the universally accepted norm against nuclear testing; a norm that has been respected by 183 countries since 1996," she said.
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