(CNN) White House Chief of Staff John Kelly said Thursday 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.

North Korea has tested over a dozen missiles since February, including its first-ever test of an intercontinental ballistic missile on July 4.

Pyongyang has said the US mainland is now within range.

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Tensions rise

North Korea and the Trump administration have exchanged a barrage of verbal volleys for months, ratcheting up the tension on the Korean peninsula and around the world as the rogue regime in Pyongyang openly threatens the United States.

Trump has mocked North Korea leader Kim Jong Un , labeling him "little Rocket Man" and promising "fire and fury" if the country continues to threaten the United States.

Most recently, North Korea's foreign minister said the President has "lit the wick" of war with his rhetoric, according to a Russian state news agency.

Kelly, a retired four-star Marine general, told reporters Thursday that North Korea is a pariah that cannot be allowed to threaten the United States.

"The American people should be concerned about a state that has developed a pretty good ICBM capability and is developing a pretty good nuclear re-entry vehicle," Kelly said in his first White House press briefing since joining the administration earlier this year.

"I would believe, I think I speak for the administration, that that state simply cannot have the ability to reach the homeland."

US officials have rarely offered a specific assessment as to North Korea's development of a reliable re-entry vehicle but in July, one official indicated that it remained a challenge.

North Korea can currently get a missile "off the ground," a lot of undetermined variables remain about guidance, reentry and the ability to hit a specific target, the official said at the time.

Kelly added: "Right now there is great concern about a lot of Americans that live in Guam. Right now we think the threat is manageable but over time, if it grows beyond where it is today, well, let's hope that diplomacy works."

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Trump's threats

Trump has used bellicose rhetoric to describe North Korea.

During his remarks at the United Nations last month, Trump threatened to "totally destroy" the rogue nation.

White House aides -- and Trump himself -- have argued that the President's blunt style is a departure from years of failed negotiations with North Korea.

"Our country has been unsuccessfully dealing with North Korea for 25 years, giving billions of dollars & getting nothing," Trump tweeted earlier this month. "Policy didn't work!"

Since Trump took power in January, the administration has been accused of sending mixed messages over the US policy on North Korea.

US Secretary of State Rex Tillerson and US Defense Secretary James Mattis attempted to present a united front in August in a co-authored opinion piece that said the US was pursing a campaign of "peaceful pressure" on North Korea.

Trump has left the door open for potential military action, saying it's not the first option but one that would be "devastating " for North Korea.

Once again, this month Tillerson stressed that the US was interested in pursuing peace through talks, however Trump hit back on Twitter saying he was "wasting his time."

I told Rex Tillerson, our wonderful Secretary of State, that he is wasting his time trying to negotiate with Little Rocket Man... — Donald J. Trump (@realDonaldTrump) October 1, 2017

"I told Rex Tillerson, our wonderful Secretary of State, that he is wasting his time trying to negotiate with Little Rocket Man..." Trump tweeted.

He continued, "...Save your energy Rex, we'll do what has to be done!"

The public display of disagreement within the White House contributed to suggestions that Trump and Tillerson's relationship was on the rocks.

Trump denied that but made it clear his opinion on North Korea is ultimately what counts.

"We actually have a very good relationship," he said, going on to concede that his views on North Korea do differ from those of his top diplomat.

"I think I have a little bit different attitude on North Korea than other people might have. And I listen to everybody," he said. "But ultimately my attitude is the one that matters, isn't it? That's the way it works."