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 United Nations 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."
North Korea agreed in principle at six-party talks in September 2005 to scrap its nuclear arms programmes in return for aid, security assurances and promises of better diplomatic ties.
Talks among the two Koreas, Japan, the United States, China and Russia were held again in November 2005 on implementing that deal, but Pyongyang has since boycotted the discussions.
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 in order 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.
Schieffer said the United States expected Japan to play a "substantial role" in implementing the UN-sponsored sanctions.
"We recognise that Japan has special considerations ... because of constitutional issues," he said.
"It's not as easy for them to participate in the sanctions regime as for others.
"I'm confident that Japan will be able to figure out a way to participate in a meaningful way that will sent a strong message to North Korea that this is not the path to take."
The UN resolution 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 nonconventional arms programmes.
Japan has already imposed sanctions including a six-month ban on all imports from the reclusive communist state, and Prime Minister Shinzo Abe said on Sunday that Tokyo was considering additional measures.