Tony Kim, a U.S. citizen and professor, was detained in North Korea on Saturday while trying to leave the country with his wife.
Haley said the United States is going to work with China to negotiate his release. Kim is at least the third U.S. citizen now detained in North Korea.
She added that China has "shown genuine concern" in regard to rising tensions with North Korea after Pyongyang tested another missile earlier this month.
"I think, really, the power has been through China. They have shown genuine concern," she said. "I think that they are trying to put the pressure on North Korea, and I think it's working."
"We're not going to do that," she said, adding that the United States will protect South Korea.
"What we can say is South Korea has been an ally from the very beginning. We want to protect them, just as we're protecting ourselves, and that's what we said we were going to do — was we were going to have the backs of our allies and we were going to call out our adversaries," Haley said.
Asked about how Trump has modified campaign positions since entering office, Haley responded, "I think it's changing with the times. I mean, the times that we're dealing with right now, we're seeing some aggression from some bad actors, we're seeing the need for strength with our allies, and we're seeing the need that we have to show force when we have to show force. And I think what we've seen the president do is say that he's not afraid to make a decision, he's not afraid to act, and he's going to change with the circumstances."