"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.
An intercontinental ballistic missile (ICBM) is a guided ballistic missile with a minimum range of 5,500 kilometres (3,400 mi) primarily designed for nuclear weapons delivery (delivering one or more thermonuclear warheads).
Similarly, conventional, chemical and biological weapons can also be delivered with varying effectiveness, but have never been deployed on ICBMs.
Most modern designs support multiple independently targetable reentry vehicles (MIRVs), allowing a single missile to carry several warheads, each of which can strike a different target.
Early ICBMs had limited precision (circular error probable) that allowed them to be used only against the largest targets such as cities.
They were seen as a "safe" basing option, one that would keep the deterrent force close to home where it would be difficult to attack.
Attacks against (especially hardened) military targets, if desired, still demanded the use of a more precise manned bomber.
This is due to the inverse-square law, which predicts that the amount of energy dispersed from a single point release of energy (such as a thermonuclear blast) dissipates by the inverse of the square of distance from the single point of release.
The result is that the power of a nuclear explosion to rupture hardened structures is greatly decreased by the distance from the impact point of the nuclear weapon.
So a near-direct hit is generally necessary, as only diminishing returns are gained by increasing bomb yield.
"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.
It was the latest in a series of claims by Pyongyang of significant new inroads in both its nuclear weapons and ballistic missile programs, although outside experts have treated a number of the claims with skepticism.
According to the North's official KCNA news agency, the ground engine test was ordered and personally monitored by the hermit nation's enigmatic leader Kim Jong-Un.
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