Lynn Pascoe made his comment in a brief airport news conference on his way to China after a four-day session in Pyongyang in another effort to persuade the communist state to rejoin stalled nuclear negotiations, Yonhap reported.
He did not elaborate.
The meeting by Pascoe, highest-ranking U.N. diplomat to visit North Korea, coincided with a trip to Beijing by North Korea's nuclear envoy to the six-party talks, which have not been held since late 2008.
North Korea has since conducted its second nuclear test and prompted the United Nations to toughen sanctions.
Ray Liotta sues skin care company over use of likeness
Aaron Carter is still in love with Hilary Duff