White House press secretary Sarah Sanders on Friday said North Korea must prove "concrete and verifiable actions" ahead of any meeting with President Trump, declining to commit to any specific date for such a gathering to take place. South Korea's national security made a surprise announcement from the White House Thursday night saying Mr. Trump has agreed to meet with North Korea's Kim Jong Un, and Kim is "committed" to denuclearization.

"Let's be very clear, the United States has made zero concessions," Sanders said from the podium Friday, adding that the U.S. must see North Korea take "concrete" actions to follow through on its promises, namely, denuclearization and halting the testing of its program.

"We have to see concrete and verifiable actions take place," she added, without giving many details about what specific steps that would entail.

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But it's unclear if Kim has specifically promised to denuclearize. South Korea's national security adviser simply said Kim is "committed" to denuclearization. Asked by CBS News' Chip Reid if Mr. Trump is giving Kim what he wants — recognition on the international stage — Sanders said that isn't the case here.

"Not at all, I think that the president is getting exactly what he wants," she said.

The unanticipated announcement that Mr. Trump has agreed to meet with Kim stunned Washington and the world Thursday night, especially since it came hours after Secretary of State Rex Tillerson said U.S. was nowhere near negotiations with Kim.

"Kim Jong Un talked about denuclearization with the South Korean Representatives, not just a freeze," Mr. Trump tweeted Thursday night. "Also, no missile testing by North Korea during this period of time. Great progress being made but sanctions will remain until an agreement is reached. Meeting being planned!"

Thursday night, U.S. and South Korean officials said such a meeting would take place by May. But on Friday, Sanders made no commitment to a timeframe. The U.S. has not yet determined the time and location of such a meeting, which Mr. Trump agreed to after speaking with South Korean officials on Thursday.