North Korean leader Kim Jong Un has "given his word" that he is committed to denuclearizing, South Korea's top diplomat said Sunday, a key condition for a potential summit with U.S. President Donald Trump planned for May.

Foreign Minister Kang Kyung-wha said in an interview aired Sunday on CBS' "Face the Nation" that South Korea has asked the North "to indicate in clear terms the commitment to denuclearization," a request she said Kim had agreed to.

"He's given his word. But the significance of his word is ... quite weighty in the sense that this is the first time that the words came directly from the North Korean supreme leader himself," Kang said. "That has never been done before."