Japan warned on Sunday it may impose harsher sanctions against North Korea if the communist state failed to meet its promise in a long-running row over its abduction of Japanese nationals.

Japan had said that it would lift some of its sanctions against North Korea after Pyongyang resumed talks with Tokyo and agreed to open a new probe to search for Japanese people kidnapped by North Korean spies during Cold War.

"We just promised to ease sanctions, believing it's going to be a sincere probe," Foreign Minister Masahiko Komura told a news programme of Japan's public broadcaster NHK.

"If we found it not sincere, (sanctions) will revive and may become a harsher one," Komura said. "If they don't move forward at all, we will take a step backward. Our stance is tongue for tongue and action for action."

Japan said it would lift restrictions on the movement of people between the two countries and end a ban on chartered flights from the impoverished state.

It also said it would allow North Korean ships to dock for humanitarian purposes, although most vessels, including a ferry that was the main link between the two countries, would remain barred.

The plight of the abductees has struck an emotional chord in Japan.

North Korea admitted in 2002 to having kidnapped 13 Japanese in the 1970s and 1980s to train its spies in Japanese language and culture.

It returned five victims and their families, then said the case was closed.