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.
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