North Korea earlier launched a previously unseen intercontinental ballistic missile, which it called a Hwasong 15, that was capable of carrying a "super-large heavy warhead" to any target in the continental United States.
US Defense Secretary Jim Mattis has said the missile flew higher than any other from North Korea, and warned that Pyongyang could soon threaten "everywhere in the world."
The US official, speaking on condition of anonymity, told AFP that America has not changed its assessment that its various missile defense systems can stop a North Korean missile attack, though the guarantee cannot be ensured indefinitely.
"I don't think they could successfully nuke the US at this time," the official said.
"There is a general sense we can stop whatever North Korea has right now. For the future, I don't know."
The United States has spent decades and billions of dollars developing technologies to stop an incoming ballistic missile, and Congress is throwing billions more dollars at the Pentagon to step up its efforts.
To protect against an ICBM, the military has the Ground-based Midcourse Defense (GMD)system, which is designed to fire an interceptor missile into space and use kinetic energy to destroy the incoming target.
America has 44 interceptors in place at Fort Greely, Alaska and Vandenberg Air Force Base in California.
It was put to the test in May, when the military successfully launched a GMD interceptor from the California base.
Checkered record
The missile blasted outside Earth's atmosphere and smashed into a dummy ICBM target, destroying it in a direct collision.
But the GMD system has had a checkered record in previous tests -- failing in earlier launches against slower-moving targets.
The official said the GMD system can shield the entire continental United States and its territories, so it is not yet necessary to install the system on the East Coast.
The GMD can stop a small number of missiles from a rogue nation but would be overwhelmed by an all-out strike from a nuclear superpower like Russia.
Such a move would likely trigger retaliatory action in what is known as Mutually Assured Destruction (MAD).
Some questions remain over the North's mastery of the technology required to guarantee any warhead would survive atmospheric re-entry -- the key element it has not yet demonstrated.
The official said that the angle of re-entry demonstrated in Wednesday's test, in which the missile went very steeply up and down, did not prove that a re-entry vehicle could survive a flight along a lower arc.
That is because the heat and friction generated by an angled re-entry into Earth's atmosphere are far greater.
The US military and its allies have other missile defense systems available, including the Terminal High Altitude Area Defense (THAAD) system capable of destroying short, medium and intermediate-range missiles in their final phase of flight.
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