"Sanctions are something that can be lifted and imposed again," Masahiko Komura said on public broadcaster NHK.

"What I want to tell the North Korean people is that they had better not underestimate or insult the United States much," he said.

"It is a big mistake if they regard it as a profit when they get what they want, and get away without delivering on what they have promised."

On Saturday North Korea agreed to completely disable its main nuclear facilities by the end of October and to allow thorough site inspections to verify that all necessary steps had been taken.

The deal was announced as the latest round of six-nation disarmament talks concluded in Beijing after a nine-month break.

In return, the other five parties -- China, Japan, Russia, South Korea and the United States -- guaranteed delivery of all heavy fuel oil promised in exchange.

Despite objections from Tokyo, Washington has already taken steps to remove North Korea from its list of state sponsors of terrorism after Pyongyang submitted a long-delayed declaration of its nuclear programmes last month.

Komura said noted the latest six-nation roundtable could not agree on specifics about a mechanism to verify the declaration.

"There will be fierce discussions on specific verification methods," Komura said.