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.

Foreign ministers from South Korea and the U.S. will more closely discuss the latest state of North Korea, Cho said, referring to an agreement made in the recent meeting in Washington between Prime Minister Yun Byung-se and Secretary of State John Kerry.

How and into what format the discussions will evolve depend on future decisions, he said, dismissing media reports that the two allies have agreed to set up a separate, multilateral channel to deal with an emergency situation in North Korea.

North Korea is believed to be facing increasing political instability following the shock execution last month of Jang Song-thaek, North Korean leader Kim Jong-un's once-powerful uncle.

In the same briefing, Cho also criticized Japan's Prime Minister Shinzo Abe for his repeated nationalist actions, including his much-denounced visit to the controversial Yasukuni shrine late last month.

Referring to Abe's recent remarks justifying his war shrine visit, Cho cited local press reports that used the saying "Praying to deaf ears" in criticizing Abe's apparent disregard for the

opinions of others.

He also warned Japan against further territorial claims to the South Korean islets of Dokdo, saying plans by Japan's Shimane Prefecture to invite Abe and other government officials to its yearly "Takeshima Day" event in February are unacceptable. Japan calls the South Korean islets Takeshima.

"It's a truly absurd act," he said, adding that "Withdrawing the unjust claims to Dokdo can only be the foundation and starting point for better Seoul-Japan ties."

The rebuke came as the bilateral relations hit the worst point after Abe's visit to the controversial shrine that honors Class-A war criminals from Japan's imperialistic past.

Paying respects at the shrine triggers angry reactions from Seoul as well as Beijing where the memories of Japan's brutality in the early 20th century still remain vivid.

It was the first Yasukuni visit by a Japanese leader in more than seven years. (Yonhap News)