The six-party forum on ending North Korea's nuclear program remains a useful tool for resolving the issue, Seoul's foreign ministry said Thursday, again urging Pyongyang to show sincerity to resume the long-stalled talks. The talks involving the two Koreas, the United States, Japan, China and Russia have not been held since late 2008 as the North was found to have continued its development of nuclear arms in breach of previous aid-for-disarmament agreements. "I feel very sorry that the dialogue is not resuming," foreign ministry spokesman Cho Tae-young said in a briefing. "It's still a useful format in resolving the North Korean nuclear issue."    In order for the talks' resumption, North Korea should first show its sincerity toward denuclearization, the spokesman said, reiterating its previous stance. The U.S., a major decision maker in the forum, has said that they will come to the table only after the North shows seriousness about giving up nuclear arms, citing broken promises.