“The president will not have the meeting without seeing concrete steps and concrete actions take place by North Korea, so the president will actually be getting something,” White House spokeswoman Sarah Sanders told a news briefing.

Sanders did not specify what actions North Korea needed to take, and a White House official later said she was not intending to set new conditions for talks with Kim.

But the comments were a sign that an end to a standoff between the two countries over North Korea‘s nuclear weapon program is not imminent.

The United States has long said it wants any talks to aim at Pyongyang abandoning its nuclear weapon and missile programs.

Hopes for a breakthrough with North Korea rose on Thursday when Trump said he was prepared to hold an unprecedented meeting with Kim.

The two leaders prompted jitters around the world last year as they exchanged bellicose insults over the North‘s attempts to develop a nuclear weapon capable of hitting the United States. Pyongyang has pursued its nuclear program in defiance of United Nations Security Council resolutions.

But tension eased around last month’s Winter Olympics in South Korea, laying the groundwork for what would be the first meeting between leaders from North Korea and the United States, and the biggest foreign policy gamble for Trump since he took office in January last year.

The head of South Korea‘s National Security Office, Chung Eui-yong, speaking in Washington on Thursday after briefing Trump about a meeting South Korean officials held with Kim this week, said the US president had agreed to meet the North Korean leader by May in response to an invitation from Kim.

Kim had “committed to denuclearization” and to suspending nuclear and missile tests, Chung said.

Some US officials and experts worry North Korea could buy time to build up and refine its nuclear arsenal if it drags out talks with Washington.

Trump’s sudden decision to meet with Kim stunned even people in his own administration.

Sanders’ comments seemed aimed at criticism in the United States that Trump took a misstep by agreeing to talks — thus giving Pyongyang the international legitimacy it seeks — but without earning any big concessions from the North.

A White House official said Trump remained committed to a meeting with Kim based on the conditions laid out by South Korea on Thursday: that Kim is committed to denuclearization, will refrain from any further nuclear or missile tests, and understands that US-South Korean military exercises must continue.

“(The North Koreans) are saying the right things and we’re going to listen to them at the table and we’re going to see where this leads,” the official said.

Jon Wolfsthal, who was a White House arms control and non-proliferation official under former President Barack Obama, said the United States should be open to a summit but only if matched with real progress on “denuclearization,” the term for North Korea giving up its nuclear weapons.

“North Korea must declare locations of nuclear material production and agree to production freeze, with verification. Without that, I worry we are being played, Kim gets his summit and we get stuck with the bill,” Wolfsthal said.

News of the planned meeting was welcomed by China, North Korea‘s largest trading partner and sole major ally, although overall trade has fallen in recent months as UN economic sanctions take effect.