"But we're very happy with how it's going with North Korea".
The US and North Korea had been at loggerheads, amid an intensifying war of nerves concerning economic sanctions against North Korea, over who should be the first to make concessions before moving forward on denuclearization talks.
But North Korea has since insisted that USA -led sanctions against it should be lifted before there will be any progress in nuclear negotiations.
The State Department said "conversations continue to take place" with North Korea, and added: "The United States remains focused on fulfilling the commitments agreed to by President Trump and Chairman Kim at the Singapore summit in June". This has fueled doubts about whether Kim will ever deal away a nuclear program he may see as his strongest guarantee of survival.
"While it's notable and disappointing that North Korea's envoy failed to show up for talks with the USA, it's not altogether surprising", Jean Lee, head of the Korea program at the Wilson Center, wrote on Twitter.
"I would love to take the sanctions off, but they have to be responsive too".
North Korea has demanded the U.S. take "corresponding measures" in return for steps it has already taken.
Kang cautioned against reading too much into the postponement, and said she would work through inter-Korean channels to urge North Korea to reschedule the delayed meeting.
"For North Korea, there has never been a USA president like Trump", Park said.
Kang said she is arranging a phone call with her United States counterpart, Pompeo. South Korea's presidential office earlier said that the meeting's postponement wouldn't affect the momentum of talks between the US and North Korea.
"We were notified by the United States that North Korea explained that (the meeting) should be postponed because both sides have busy schedules", Kang said. The State Department said the talks would be rescheduled "when our respective schedules permit".
Trump told a White House news conference the change was "because of trips that are being made", but did not elaborate on these. He did not provide a straightforward answer when asked whether the discord over sanctions has made it more hard to set up meetings. "This has to do with timing as a matter - we're talking about scheduling".
The US has been pushing North Korea to disclose an inventory of its nuclear weapons program and allow US inspectors to verify the dismantlement of its Punggye-ri nuclear site, its Tongchang-ri missile engine test site and possibly its Yongbyon nuclear complex.
Kim Jong Un shifted to diplomacy early this year and has held three summits with South Korean President Moon Jae-in.
Kim pledged to work towards denuclearization at an unprecedented first meeting with Trump in Singapore but negotiations have made little headway since, with North Korea falling short of USA demands for irreversible moves to abandon a weapons program that potentially threatens the United States.