North Korea will have Guam attack plan ready by mid-August, military statement says

President Donald Trump declared the U.S. nuclear arsenal "far stronger and more powerful than ever before," even as his top diplomat was working to calm the North Korea crisis and insisting there wasn't "any imminent threat."

In a series of early-morning tweets Wednesday, Trump reaffirmed his threat from a day earlier by reposting video of him warning the previous day that Pyongyang would be "met with fire and fury like the world has never seen" if it made more threats to the U.S. Then, he said that his first order as president had been to "renovate and modernize" the U.S. nuclear arsenal.

"Hopefully we will never have to use this power, but there will never be a time that we are not the most powerful nation in the world!" Trump tweeted.

It wasn't immediately clear what evidence the president had, if any, to support his claim about the nuclear force.

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The crisis centres on comments from North Korea's army, which said it is studying a plan to create an "enveloping fire" in areas around Guam, a U.S. territory about 3,400 kilometres away, with medium- to long-range ballistic missiles.

Trump issued an executive order in his first days in office calling for a review to ensure the U.S. nuclear deterrent is "modern, robust, flexible, resilient, ready" and appropriately tailored for 21st century threats. The White House has not detailed any findings from that evaluation.

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A modernization effort started by former president Barack Obama is in the early stages, but the force is essentially unchanged from the way Trump inherited it on Jan. 20.

Only hours before Trump's tweets, Secretary of State Rex Tillerson urged calm and said Americans should have "no concerns" despite the exchange of threats between the president and North Korea. Aboard his plane as he flew home from a working trip to Asia, Tillerson insisted the developments didn't suggest the U.S. was moving closer to a military option to dealing with the crisis.

"Americans should sleep well at night," Tillerson said.

"Nothing that I have seen and nothing that I know of would indicate that the situation has dramatically changed in the last 24 hours."

Tillerson said the U.S. would defend itself and its allies from any provocations.

"What the president is doing is sending a strong message to North Korea in language that Kim Jong-un can understand, because he doesn't seem to understand diplomatic language," he said.

Guam in the crosshairs

The comments put Tillerson once again in the role of translating the president's aggressive rhetoric into more diplomatic terms, and of working to minimize the chances of public panic. In fact, Tillerson argued that North Korea's escalating threats indicated it was feeling the pressure from a successful U.S. strategy.

The different tones struck by Trump and Tillerson put the onus on the North Koreans to decide how to interpret the latest missives from the U.S.

Heather Nauert, spokesperson from the State Department, denied in a press briefing Wednesday that mixed messages were being sent from within the administration, saying, "we are all singing from the same hymn book."

The New York Times has reported that senior officials were not prepared for the precise language Trump would use Tuesday as he answered a question while preparing to meet with officials about escalating opioid drug use in the U.S.

Nauert said the focus should not be on Trump's language but the provocative actions from North Korea that are alarming the international community.  "Our issue is not with the people of the DPRK, but with the regime itself," she added.

In response to Trump's "fire and fury" comments, North Korea threatened to hit Guam with its Hwasong-12 missiles, which it says can carry a heavy nuclear warhead. The tiny U.S. territory houses U.S. military bases and is a common refuelling stop for U.S. government aircraft traversing the Pacific Ocean.

In a statement later carried by state-run news agency KCNA, North Korea's military said it will complete a plan for the attack by mid-August. This plan for the "historic enveloping fire at Guam" will then be conveyed to the commander-in-chief of its nuclear force and then "wait for his order," the statement said.

Calling Trump "a guy bereft of reason" and his comments a "load of nonsense," the statement said North Korea will "keep closely watching the speech and behaviour of the U.S."

Todd Thompson, a lawyer who lives on Guam, said he laughed off past threats because he "figured cooler heads in Washington would prevail, and it was just an idle threat."

"But I have to say, I'm not laughing now," Thompson said. "My concern is that things have changed in Washington, and who knows what's going to happen?"

Trump threat not a bluff: Graham

Though Tillerson insisted there was no imminent threat, he noted even if there were, "the North Korean missile capability can point in many directions, so Guam is not the only place that would be under threat."

U.S. Defence Secretary James Mattis weighed in publicly on Wednesday, urging North Korea to "stand down in its pursuit of nuclear weapons."

"The DPRK regime's actions will continue to be grossly overmatched by ours and would lose any arms race or conflict it initiates," Mattis said in a statement.

At least one prominent lawmaker felt Trump wasn't bluffing with his threat.

Sen. Lindsey Graham, South Carolina Republican on the Senate's armed services committee, told CBS's This Morning that Trump had "basically drawn a red line" by saying Pyongyang can't ever have a nuclear-tipped intercontinental ballistic missile capable of striking the U.S.

"He's not going to let that happen," Graham said. "He's not going to contain the threat. He's going to stop the threat."

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Tillerson, who spent the past days in Asia working the North Korea conflict, said he didn't believe a new diplomatic strategy was needed. To the contrary, he said the latest threat from the North suggested the current strategy was working. After months of frustration over China's reluctance to pressure Pyongyang economically, the U.S. on Saturday secured a unanimous UN Security Council vote to authorize sweeping new sanctions that target one-third of the North's exports.

"The pressure is starting to show," Tillerson said. "I think that's why the rhetoric coming out of Pyongyang is beginning to become louder and more threatening. Whether we've got them backed into a corner or not is difficult to say, but diplomatically, you never like to have someone in a corner without a way for them to get out."

To that end, Tillerson said there was still an off-ramp available to Pyongyang: A return to negotiations with the U.S., a step that Tillerson has previously said can happen only if Kim's government gives up its nuclear aspirations, starting with an extended pause in missile tests.

"Talks," Tillerson said when asked if North Korea had a way out. "Talks, with the right expectation of what those talks will be about."

In Pyongyang on Wednesday, tens of thousands packed Kim Il-sung Square for a rally planned after the UN sanctions that followed a familiar format of speeches from a balcony, with the crowd listening below, standing in organized rows interspersed with placards and slogans.