U.S. Defense Secretary Mark Esper says he does not regret postponing a U.S.-South Korean military air exercise, even though the gesture was rejected by North Korea as not enough to restart nuclear diplomacy.

Esper spoke to reporters Thursday morning shortly before boarding his plane in Hanoi, Vietnam, for a flight to Washington.

He described North Korea’s response to the offer as being “not as positive as we would have liked.”

North Korea says the U.S. must scrap the drill entirely and abandon its hostility if it wants nuclear negotiations to resume.

Esper also said he does not believe there is a rift in the U.S.-South Korean alliance after negotiations faltered this week over a U.S. demand to significantly increase what Seoul pays to keep 28,500 American troops on its soil.