Although President Donald Trump recently declared his top diplomat was "wasting his time" trying to negotiate with the North, Kelly said he hopes diplomacy works before the nation can develop its weapons capabilities further.
Kelly comments at a White House news conference were milder than Trump's recent pronouncements. Over the past three weeks the president has exchanged threats and personal insults with North Korean leader Kim Jong Un, escalating tensions.
Then he spelled out a bottom line.
"I think I speak for the administration, that that state can simply not have the ability to reach the homeland," Kelly said.
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He said there's already "great concern" about Americans who live in Guam. North Korea in August threatened to fire a salvo of intermediate range missiles toward the US Pacific island territory, a major military hub and home to US bombers that periodically fly missions over the divided Korean Peninsula.
"Let's hope that diplomacy works," he said.
Trump, on Wednesday, conceded differences on North Korea with Secretary of State Rex Tillerson, who advocates keeping open the possibility of negotiations with Kim's authoritarian government.
"I think perhaps I feel stronger and tougher on that subject than other people," Trump said in the Oval Office.
"But I listen to everybody and ultimately I will do what's right for the United States and really what's right for the world," he said, adding, "it's a problem that has to be solved."
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