Senior officials in Seoul say it will be a matter of "serious concern" if Pyongyang does
reprocess more spent nuclear fuel into weapons material. They also say that referring the issue to the United Nations Security Council is not out of the question.
South Korean officials say they hope diplomacy works to persuade North Korea to give up its nuclear weapons programs.
However, on Wednesday, South Korean Foreign Ministry spokesman Kim Sung-chul said Seoul does not rule out taking the issue to the United Nations Security Council, if talks do not work.
Mr. Kim said Seoul would be willing to discuss a Security Council referral only as part of a much broader look at strategic issues, if there are no results from talks aimed at ending North Korea's nuclear weapons programs.
His comments come as South Korean officials grow concerned about evidence that North Korea has shut down its Yongbyon nuclear reactor, possibly to unload used fuel rods and reprocess them to make weapons.
South Korean Foreign Minister Ban Ki-Moon says it would be a "matter of serious concern" if North Korea were in fact reprocessing fuel. He also said Seoul and the United States are not discussing taking the issue to the Security Council.
The council could impose sanctions on North Korea for violating its previous agreements to remain free of nuclear weapons. Pyongyang has repeatedly said it would consider sanctions to be an act of war.
The United States has been working since last June with Japan, China, South Korea, and Russia to persuade the North to return to nuclear talks. Pyongyang says it already has nuclear weapons and plans to make more, and has suspended its participation in the talks.
In recent weeks, U.S. officials have indicated they could not wait indefinitely for North Korea to return to talks, and that Washington may consider other options to resolve the issue. What those options are has never been fully laid out.
Professor Ha Yong-chul, an East Asia specialist at Seoul National University, says a referral to the Security Council would likely be a serious escalation of the nuclear dispute.
"I think it's the first step toward some form of sanctions, whether it's economic blockade, or military action. I think the six-party formula would be over," he said.
Professor Ha says a Security Council referral would increase the influence of the United States in the matter, while diminishing that of South Korea. He says China would also lose influence, because it would have failed to resolve the matter through talks, despite being North Korea's only major ally.
In 1993 and 2003, the Security Council expressed its concern and urged North Korea to comply with its past commitments. It has never voted to impose sanctions on the North over the nuclear issue.
The United States and its regional partners have not set any formal deadlines for North Korea to return to the six-way talks. Pyongyang says it needs a nuclear deterrent against what it describes as a hostile attitude by the United States, despite repeated assurances from Washington it has no intention to attack the North.