(CNN) US President Donald Trump's decision to meet with North Korean leader Kim Jong Un "was not a surprise in any way," US Secretary of State Rex Tillerson said Friday, less than a day after the top US diplomat said conditions were not yet ripe for negotiations with North Korea.

Speaking to reporters in Djibouti, the latest stop on his week-long tour of Africa, Tillerson explained the sudden shift by saying it was "a decision the President took himself" after having the matter "on his mind for quite some time." It followed a "fairly dramatic" change in posture from Kim, he added.

The bombshell news , announced by a South Korean delegation at the White House on Thursday night, was met with wary optimism by China and US allies in the region.

There's no word yet on who will attend the meeting -- the first between a sitting US President and a North Korean leader -- aside from Trump and Kim.

One obstacle the US State Department faces ahead of talks is a lack of senior diplomats experienced in dealing with North Korea. The Trump administration has yet to appoint a permanent ambassador to South Korea and its top North Korea diplomat, Joseph Yun, announced his surprise retirement last month.

A senior State Department official told CNN "we'll be ready" for the talks to advance. The White House and National Security Council will take the lead on talks at this time, the official added, with the latter expected to meet with the State Department soon to game out a strategy.

The breakthrough brings the North Korean regime close to its long-desired aim of recognition on the international stage, and offers Trump the tantalizing prospect of a historic diplomatic victory, although the consequences of such a high-stakes gamble remain hard to predict.

US allies in the region have given a cautious welcome to the development while vowing to keep up intense diplomatic and economic pressure on North Korea.

On a phone call Friday with Trump, Chinese President Xi Jinping said he hoped the US and North Korea would start dialogue as soon as possible and strive for positive results, China's state-run Xinhua news agency said.

Earlier Friday, Geng Shuang, a spokesman at China's Ministry of Foreign Affairs, described the development as positive and said it was a moment to show "political courage." But he stressed that China would continue to maintain sanctions on North Korea until a political settlement was reached.

Japanese Prime Minister Shinzo Abe talked to Trump and Tillerson on the phone to reiterate the necessity of maintaining pressure on North Korea.

Abe, briefing reporters after the call, said that the US and Japan had agreed to "keep putting maximum pressure until North Korea takes concrete actions toward denuclearization."

Australian Foreign Minister Julie Bishop said her government welcomed "any dialogue with North Korea" but warned that "North Korea has a history of making agreements and then failing to honor them."

"The dialogue must ensure North Korea abides by numerous United Nations Security Council Resolutions banning its nuclear weapons and ballistic missile programs," she said.

No North Korean response

South Korea's President Moon described the announcement as "historic" and thanked both leaders for seeking a diplomatic solution to the denuclearization of the Korean Peninsula.

South Korea's national security adviser, Chung Eui-yong, who delivered the invitation to Trump in Washington after a visit by his delegation to Pyongyang earlier this week, was careful to praise Trump's influence over the developments. Chung said the US President's "leadership" and his administration's pressure on the North Korean regime had "brought us to this juncture."

Some observers noted that even while the South Korean delegation was in town, Trump signed off on his plan to impose steep tariffs on steel and aluminum imports -- a move that could impact precisely those allies involved in keeping up the pressure on North Korea.

North Korea has not yet responded to the news of Trump's decision to meet with Kim. The development has also not yet made it into news bulletins on state media.

Russian Foreign Minister Sergey Lavrov said Moscow hoped the meeting would take place, according to Russia's state-run Tass news agency.

"We consider it to be a step in the right direction. Hopefully, the agreement will be implemented," he said, while on a trip to Ethiopia.

Changing times

Hours before the news broke, Tillerson had said direct talks between the US and North Korea were still in the distant future.

"We're a long way from negotiations. We just need to be very clear-eyed and realistic about it," Tillerson said from Ethiopia on Thursday. "I don't know yet, until we are able to meet ourselves face to face with representatives of North Korea, whether the conditions are right to even begin thinking about negotiations."

A day later, the top US diplomat insisted the United States had been "saying for some time that we're open to talks" with the North Koreans and that Trump's decision was "no surprise," coming on the heels of recent intra-Korean dialogue and contacts through other channels.

He also sought to draw a line between talks and negotiations, saying: "With respect to talks with North Korea versus negotiations -- and I think this seems to be something that people continue to struggle with the difference -- my comments have been that the conditions are not right for negotiations."

What was a surprise was Kim's change in stance when he met with the South Korean delegation, when the North Korean leader displayed "not just willingness but his strong desire for talks," said Tillerson.

"So I think really what changed was his posture, in a fairly dramatic way in all honesty, that came as a little bit of a surprise to us as well that he was so forward leaning in his conversations with the delegation from South Korea," he said.

Vice President Mike Pence said Kim's willingness to meet was "evidence that President Trump's strategy to isolate the Kim regime is working" and that it was happening "despite the United States making zero concessions."

The stunning announcement was the culmination of a diplomatic whirlwind that began with the invitation of a North Korean delegation to attend the Winter Olympics last month in Pyeongchang, South Korea.

Sanctions to remain in place

There are many details to be ironed out before any Trump-Kim meeting could take place, not least the location.

The Panmunjom truce village in the Korean Demilitarized Zone (DMZ), one possible venue, hosted meetings between North and South Korea in the run-up to the Winter Olympics.

The South Koreans said the talks could take place as soon as May; Tillerson said it would take "some weeks" to agree on the timing and the location of the meeting.

Since Trump came into office, the US has leveled some of its most significant and far-reaching sanctions against North Korea and has also succeeded in pressuring China to further isolate the regime.

That pressure would not abate as the US heads toward the historic talks, the White House said.