US Ambassador to Japan Thomas Schieffer said on Monday a return by North Korea to stalled six-party talks on its nuclear programme would not be sufficient to end UN sanctions.
"A return to six-party talks kind of doesn't do it," Schieffer said. "You have to come to the six-party talks and agree on how you are going to implement the September 19 agreement.
"If that implementation could then be verified by the international community, I think you would see walking back from the sanctions regime," he said.
But he added: "This is long way to go."
A weekend UN Security Council resolution imposing financial and weapons sanctions on North Korea for its claimed nuclear test allows nations to stop cargo going to and from North Korea to check for weapons of mass destruction or related supplies.
Japan is now debating how it can take part in such inspections within the scope of its pacifist constitution.
The resolution also bars trade with North Korea in dangerous weapons, imposes bans on heavy conventional weapons and luxury goods, and asks nations to freeze funds connected with North Korea's non-conventional arms programmes.