It is in North Korea's interests to carry out its pledge to complete a probe into the abduction of Japanese nationals, South Korea's chief delegate to the six-party talks on the North's denuclearization said Tuesday.

Kim Sook, who met in Tokyo with his Japanese counterpart, Akitaka Saiki, told reporters that Pyongyang must abide by last week's agreement with Japan, which would set the stage for Tokyo to take part in six-party energy aid for the communist state.

"Progress must be made" on resolving the abduction issue to ensure Japan's involvement in providing the energy assistance, he said, adding that Seoul recognizes the gravity of the abduction issue for the Japanese public.

A senior Foreign Ministry official in charge of North Korean issues told reporters that Kim was briefed on last week's working-level negotiations between Japan and North Korea, where Pyongyang agreed to reopen and complete by this fall reinvestigations of abductees in exchange for Tokyo's partial lifting of economic sanctions.

Kim in return shared discussions he had last week with U.S. Assistant Secretary of State Christopher Hill over the progress of six-party talks.

"Japan and South Korea agreed that bilateral cooperation is vital in advancing the six-party talks and denuclearization of North Korea," the senior Foreign Ministry official said.

Kim acknowledged to reporters that progress has been sluggish but expressed hope North Korea will take action by the fall.

The six parties have produced an agreement to lift sanctions in exchange for nuclear disarmament, but the U.S. has kept North Korea on its list of terrorism-sponsoring countries.