Washington (CNN) A North Korean official reaffirmed Pyongyang's commitment to developing a long-range intercontinental ballistic missile capable of reaching "all the way to the East coast of the mainland US," on Monday, telling CNN that the rogue nation is currently not interested in diplomacy with the US until it achieves that goal.

The comments offer a sobering challenge to the US and the Trump administration which has offered mixed messages regarding diplomatic efforts with North Korea.

White House chief of staff John Kelly said last week that Americans should be concerned about North Korea's ability to reach the United States with an intercontinental ballistic missile, cryptically telling reporters that if the threat grows "beyond where it is today, well, let's hope that diplomacy works."

Significantly, Kelly noted that Pyongyang "is developing a pretty good nuclear re-entry vehicle."

For a missile to successfully strike a target it would have to re-enter the earth's atmosphere without breaking up.

Kelly's comments seem to indicate that the US believes that North Korea is close to achieving what would be a key breakthrough for their missile program.

The North Korean official told CNN that two additional steps are needed to achieve its goal of a reliable ICBM.

One step is an above-ground nuclear detonation , like the kind North Korean Foreign Minister Ri Yong Ho threatened at the United Nations last month when he suggested leader Kim Jong Un was considering detonating "an unprecedented scale hydrogen bomb " over the Pacific after Trump threatened to "totally destroy" North Korea.

The other step is the "testing of a long-range ICBM capable of reaching Guam - and even further," the official said.

Last week, North Korean state media renewed a threat to launch missiles toward the US territory of Guam, warning that "reckless moves" by the US would compel Pyongyang to take action.

North Korea first said it was examining a plan to target the Pacific island in August after Trump warned the isolated regime would "face fire and fury like the world has never seen" following a US intelligence assessment that North Korea had produced a miniaturized nuclear warhead.

Both steps are necessary for the DPRK to send that "clear message" to the Trump administration that it possesses an effective nuclear deterrent, the North Korean official said on Monday.

The official added that one or both of these tests could possibly be timed to coincide with the US-South Korea joint naval drills, which began Monday and will last for ten days, or could possibly coincide with Trump's visit to Asia next month.

Joint military exercises are particularly infuriating to Pyongyang. The North Korean government views them as a dress rehearsal for an invasion -- even as the US insists they are purely defensive in nature.

The US will practice evacuating civilians from South Korea in its annual Courageous Channel training exercise later this month -- a routinely scheduled exercise that "prepares service members and their families to respond to a wide range of crisis management events such as non-combatant evacuation and natural or man made disasters," according to US Forces Korea.

Asked Monday if his possible visit to the Korean Demilitarized Zone (DMZ) next month will provoke North Korea Trump said he would "take a look at that" but added he "hadn't heard in terms of provoking."

On Sunday, US Secretary of State Rex Tillerson said his diplomatic efforts will continue as long as they possibly can despite the saber-rattling on both sides of the Pacific.

"Those diplomatic efforts will continue until the first bomb drops," Tillerson said on CNN's "State of the Union."

Democratic Rep. Brendan Boyle of the House Foreign Affairs Committee told CNN's Wolf Blitzer on Monday that he believes the US should continue to pursue diplomacy with Pyongyang but ultimately, must prevent North Korea from developing a nuclear weapon capable of reaching the US.

"I don't believe we can live with that possibility," he said.

North Korean anger over US 'decapitation' plan

A North Korean diplomat publicly expressed anger on Monday over reports that hackers gained access to a US-South Korean plan to "decapitate" the regime's leadership.

North Korea's deputy UN Ambassador warned that the plan is "dangerous" and an insult to the nation.

CNN has reported that the North Korea hackers stole classified military documents from a South Korean Defense ministry database in September 2016, according to Rhee Cheol-hee, a member of South Korea's National Assembly. He said the documents stolen included the South Korea-US wartime operational plan and a document that includes procedures to "decapitate" the North Korean leadership.

North Korea's deputy Ambassador Kim In Ryong told a UN committee meeting on nuclear issues that "what is more dangerous is that the US dared to formulate a plan and stage the exercise of decapitation operation and secret operation aimed at the removal of our supreme leadership."

"This is an unbearable insult to the supreme dignity of the DPRK and it arouses extreme anger from our people and service personnel," Ryong said.

The North Korean UN diplomat also fiercely defended Pyongyang's nuclear and missile tests.

Why North Korea wants nukes and missiles North Korea has long maintained it wants nuclear weapons and long-range missiles to deter the United States from attempting to overthrow the regime of Kim Jong Un. Pyongyang looks at states such as Iraq -- where Saddam Hussein was overthrown by the United States -- and Libya -- its late leader, Moammar Gadhafi, gave up his nuclear ambitions for sanctions relief and aid, only to be toppled and killed after the United States intervened in his country's civil unrest -- and believes that only being able to threaten the US mainland with a retaliatory nuclear strike can stop American military intervention. Many experts say they believe North Korea would not use the weapons first. Kim values his regime's survival above all else and knows the use of a nuclear weapon would start a war he could not win, analysts say.

"Unless the hostile policy and the nuclear threat of the US is thoroughly eradicated, we will never put our nuclear weapons and ballistic rockets on the negotiation table under any circumstance and will never flinch even an inch from the road we have chosen, upholding the line of simultaneously developing the two fronts, the everlasting banner in safeguarding the peace," Ryong said.

"This year, we have passed the final gate of completing the state nuclear force and thus became the full- fledged nuclear power which possesses the delivery means of various ranges, including the atomic bomb, H-bomb and intercontinental ballistic rockets," he said. "The entire US mainland is within our firing range and if the US dares to invade our sacred territory even an inch it will not escape our severe punishment in any part of the globe."

Mixed messages

US-led efforts to apply additional diplomatic pressure on North Korea in recent months have been met with greater defiance as the Kim regime continues to march toward realizing its nuclear ambitions.

Despite assurances from Defense Secretary James Mattis and Secretary of State Rex Tillerson that the US continues to seek a peaceful resolution, neither side has overtly indicated that it is ready or willing to engage in serious negotiations.

Trump has repeatedly undercut comments made by his top officials and publicly engaged in a war of words with Kim that has devolved into insults and name-calling -- only raising fears of an eventual military conflict.

Trump said earlier this month that the US will "do what we must do" to prevent further threats from North Korea.

"We cannot allow this dictatorship to threaten our nation or our allies with unimaginable loss of life," he said at a meeting with top military officers. "We will do what we must do to prevent that from happening and it will be done if necessary, believe me."