(CNN) North Korea is working on an advanced version of its existing KN-20 intercontinental ballistic missile that could potentially reach the United States, less than six months after it launched its first ICBM, a US official tells CNN.

This newly assessed ICBM is just one part of an accelerated effort by Kim Jong Un's regime to improve every part of its missile and nuclear weapons attack capability as quickly as possible, according to several US officials familiar with the North Korean program.

All of these improvements are forcing the US to recalculate the timing of the threat that a North Korean missile could pose to the US, and the need to continuously update US military response options.

The officials spoke to CNN on the basis of anonymity because of the sensitivity of the issue, but all strongly emphasized that they were not discussing classified information.

Additional new improvements are underway to North Korea's nuclear fuel, missile launchers, guidance and targeting systems as well, officials say. All of this comes as President Donald Trump is about to travel to Asia , where North Korea's weapons will be a major topic of discussion.

The US continues to believe it may be possible that during 2018, North Korea will be able to take the critical step of putting a miniaturized warhead on top of an intercontinental missile, officials say.

With the new year just less than eight weeks away, the administration continues to look at fine-tuning military response options. While at the same time by the end of 2017, the Pentagon is expected to finish both a so-called Nuclear Posture Review and a Ballistic Missile Defense Review that will set the stage for billions of dollars and decades of new spending on modernized US weaponry to counter the type of threat North Korea poses.

The officials say the accelerated North Korean program is a major reason why so many senior Trump administration officials are publicly talking about a more worrisome threat even as they continue to pin hope on a diplomatic solution with the North. "In terms of a sense of urgency, today, North Korea certainly poses the greatest threat," Gen. Joseph Dunford, chairman of the Joint Chiefs of Staff, recently testified to Congress.

Defense Secretary James Mattis has also been sounding the alarm, noting recently, "North Korea has accelerated the threat that it poses to its neighbors and the world through its illegal and unnecessary missile and nuclear weapons programs," he added, emphasizing that the US "does not accept a nuclear North Korea."

But all the officials CNN spoke with acknowledge that the US is already confronting a nuclear North Korea, because they have carried out a number of nuclear and missile tests. Officials say the dilemma for US intelligence is to know if a North Korean missile is on a launcher and whether it is armed with a nuclear warhead, and is ready to fire.

Despite rhetoric from the White House, inside military and intelligence circles there is little appetite for a pre-emptive strike against North Korean weapons sites, although the US clearly retains that option. But there is a firm view that a nuclear-tipped missile launch would not be allowed to proceed. There will be likely be intelligence indicators from a launch site if that were about to happen, but the indicators may not be foolproof, officials say.

The improvements that North Korea is making that indicate the threat is more urgent include: improved solid rocket fuel with more stability that burns hotter and more evenly to help the missile achieve greater distances; improved rocket motors and engine components, and improved targeting and guidance systems.

North Korea also appears to have made critical progress in developing warheads that will be capable of re-entering the Earth's atmosphere and reaching a target at intercontinental ranges. At least two recent ballistic missile tests demonstrated a dummy warhead successfully re-entering the atmosphere at least in a limited fashion, by not shattering.

This means the dummy test warhead separated successfully from the body of the missile and largely survived the massive heat and friction stress of re-entering the atmosphere after being fired into high altitudes.

North Korea is also assumed by US intelligence to have a functioning warhead they have never tested. Any such test could be perceived as a threat.

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One area where North Korea may be running into serious problems is at its underground nuclear test site at Punggye-ri. US officials are for now dismissing regional press reports that one tunnel completely collapsed, killing hundreds of North Korean workers.

But one US official said that US intelligence is watching the site closely to try to assess what damage may have recently occurred. North Korea conducted a massive underground nuclear test at the site in September and since then there have been several seismic events that geological experts have publicly said make further underground collapse likely due to the shock waves of the test.