U.S. President Donald Trump will not meet with North Korean leader Kim Jong Un unless he takes "concrete and verifiable actions" to meet promises he has made, the White House said Friday.

"We're not going to have this meeting take place until we see concrete actions that match the words and the rhetoric of North Korea," press secretary Sarah Huckabee Sanders said at a press briefing. "We have to see concrete and verifiable actions take place."

Sanders did not specify what the "concrete and verifiable actions" would be, but she was referring to Kim's messages conveyed Thursday to Trump via a South Korean official that the North's leader is committed to denuclearization, will refrain from any further nuclear and missile tests, and recognizes regular military exercises between the United States and South Korea will continue.

Hopes for a breakthrough on the North Korean nuclear issue rose on Thursday when Trump agreed to meet with Kim by May for a first-ever bilateral summit. The two countries have no diplomatic ties.

Sanders said the time and location of the summit have yet to be determined. Secretary of State Rex Tillerson, speaking to journalists in Djibouti in Africa, said it will take "some weeks before we get all that worked out."

Sanders said the United States has made no concessions in accepting an invitation from Kim to meet Trump. She added Washington will maintain its policy of imposing "maximum pressure" with strong sanctions on Pyongyang to compel it to give up its nuclear weapons.

"We're going to continue the maximum pressure campaign," she said. "We're going to continue working with our allies and partners to do that, and we're going to continue to ask them to step up and do more."

Speaking by phone Friday, Trump and Chinese President Xi Jinping affirmed they remain committed to "maintain pressure and sanctions until North Korea takes tangible steps toward complete, verifiable, and irreversible denuclearization," according to the White House.