"North Korea is acting in an unsafe way not only toward Japan, China and South Korea, but the entire world. North Korea's actions are not good for anyone and certainly not good for North Korea," Sarah Sanders, the President's press secretary, said in an email.

But Tillerson's comments met with approval in at least one part of the world, Russia.

"We can say that such constructive statements are much more appealing than the confrontational rhetoric which we have heard so far," Kremlin spokesman Dmitry Peskov told reporters Wednesday.

Speaking during a conference call Peskov said Tillerson's stance is "welcomed, and most importantly, is in line with appeals from Russia. As President [Vladimir] Putin has repeatedly said, confrontational rhetoric and any steps that can provoke further tension on the Korean peninsula are absolutely counterproductive. It was Putin who traditionally and consistently urged all interested parties to do everything possible to build channels for dialogue, therefore, such statements satisfy us."

Tillerson had previously said that the United States was willing to negotiate, but has been vague about what would need to happen before both sides sat down together and whether talks would be subject to certain preconditions.

Kingston Reif, the director for disarmament and threat reduction policy at the Arms Control Association, told CNN that Tillerson's comments were the most detailed to date on the potential parameters for talks.

"He made clear that initial meeting(s) can begin without preconditions and without North Korea committing to denuclearization. He also said that talks would be difficult to sustain in absence of North Korea's testing freeze," Reif said.

Trump's take

"Key questions remain," Reif said of Tillerson's latest comments. "Does Tillerson speak for the administration? And what would we be willing to put on the table in return for sustaining a testing freeze and more comprehensive talks?"

US National Security Adviser HR McMaster, one of Trump's most trusted foreign policy aides, said Tuesday that the global community must "make the most out of what might be our last best chance" to avoid military action, echoing comments he previously made that "time is running out."

Those comments may be more indicative of the administration's actual policy on North Korea, as Tillerson is believed to have been marginalized in the policy-making process. Reports surfaced last month that the Trump administration was considering replacing Tillerson with CIA Director Mike Pompeo, though the White House pushed back when asked about the rumors.

"You need both sides to be credible in the good cop-bad cop routine for it to work. There are too many doubts about the weight and credibility of Tillerson." said Vipin Narang, an associate professor of political science at MIT and a member of MIT's Security Studies Program. "McMaster has the ear of the President and is much more credible."

Demands

Tillerson said it wasn't workable to demand that North Korea give up its weapons before talks begin, and that US President Donald Trump agrees. The administration has said that serious negotiations with North Korea will have to focus on denuclearization.

"It's not realistic to say we are only going to talk if you come to the table ready to give up your program," Tillerson said. "They have too much invested in it, and the President is very realistic about that as well."

The top US diplomat laid out one caveat: North Korea would have to ensure a period of quiet during talks.

"So I clearly think they understand that if we are going to talk, we have to have period of quiet ... or it's going to be very difficult to have productive discussions. And so we continue to indicate to them, we need a period of quiet. You need to tell us you want to talk, the door is open. But we'll show up when you tell us you're ready to talk," he said.

Responding to Tillerson's comments Wednesday, Chinese foreign ministry spokesman Lu Kang welcomed all efforts that would ease tensions, and reiterated China's willingness to "play a constructive role in promoting a peaceful resolution to the issue through dialogues."

"China has raised its 'dual-track thought' and 'suspension for suspension' proposal. We hope to see the United States and North Korea move in the same direction, and make meaningful steps towards dialogues and contact," said Lu.

Though some North Korea analysts agree that mandating that North Korea give up its nuclear weapons up-front is a doomed strategy, others worry that Pyongyang's history of violating international agreements makes negotiation without the preconditions too risky.

"The Trump administration should insist that North Korea commit to denuclearization upfront to avoid the failed negotiations of the past and any testing freeze is accompanied by verification that the programs do not continue during the negotiations," Anthony Ruggiero, a former US State Department and Treasury Department official, said in a tweet.

"In my professional career this was certainly the most important mission I've ever undertaken," Feltman, a former State Department veteran, said Tuesday.

When asked about Tillerson's comments on preconditions, Feltman said he told his North Korean hosts that "we really believe that they need to signal that they're willing now to go in a different direction, to start some kind of engagement, to start talking about talks. And I think they listened seriously to our arguments."

"But they did not offer any time of commitment to us at that point. I think that they have to reflect on what we said with their own leadership and I don't want to try to suggest that I can read the minds of my interlocutors or put words in their mouth," Feltman said.

'Ready to talk anytime'

Tillerson coupled the public invitation to Pyongyang with an update on the international pressure campaign he has helped build that has squeezed North Korean revenues from coal, textiles and forced labor, winnowed down its oil imports, and left it increasingly isolated on the diplomatic stage as countries across the globe expel its diplomats.

Speaking earlier in the day at the State Department, Tillerson told employees there that the US will maintain the diplomatic campaign "and continue to turn that pressure up until we can get an engagement in a meaningful way with North Korea."

"We need to have DPRK come to the table," Tillerson added at the Atlantic Council. "We're ready to talk anytime they're ready to talk, but they have to come to the table and they have to come to the table with a view that they're ready to make a different choice."

Tillerson said that he has told Mattis that if diplomacy doesn't work, he will have "failed." He added that diplomatic efforts are backed by military options and that if North Korea makes "bad choices," the US is ready.

"We have a strong military presence standing behind us," Tillerson said of his diplomatic efforts. "If North Korea makes bad choices, we're prepared."

At the Atlantic Council, Tillerson said the question was about how to "begin the process of engagement, because we are dealing with a new leader in North Korea who no one has ever engaged with."

"The important thing is that we get started," he added.

This article has been updated.