SEOUL, South Korea -- South Korea on Monday promised a major new proposal if North Korea returns to six-nation disarmament negotiations as the rivals began two days of their first face-to-face talks in 10 months amid heightened tensions over Pyongyang's nuclear ambitions.
The resumption of dialogue between the two countries was the first potentially positive development on the Korean Peninsula since February, when North Korea claimed it had nuclear weapons and said it would indefinitely boycott arms talks until Washington drops its "hostile" policy.
Pyongyang, with a history of brinksmanship to wring aid and other concessions from the West, said last week it had completed removing spent fuel rods from a reactor at its main nuclear complex -- a process that could allow it to harvest more weapons-grade plutonium -- and would strengthen its nuclear arsenal.
The North Korean delegation listened without comment as South Korea's Vice Unification Minister Rhee Bong-jo brought up the nuclear issue during Monday's first session in the North Korean border town of Kaesong. Pyongyang normally shuns direct talks with the South over its atomic program.
"If the six-party talks resume, it shouldn't be talks for the sake of talks, but substantial progress is necessary," Rhee said. "For this, the South side is preparing for a substantial proposal, and will propose it to the related countries when the talks resume." He didn't elaborate.
Secretary of State Condoleezza Rice, meanwhile, warned North Korea against testing nuclear weapons.
"Escalation on the part of the North Koreans is going to deepen their isolation a lot," she told reporters Monday after a visit to Iraq. She did not elaborate.
National Security Adviser Stephen Hadley threatened unspecified actions against North Korea if it carried out a nuclear test, a position echoed by Japan.
"We've seen some evidence that says that they may be preparing for a nuclear test," Hadley told CNN. "Obviously, that would be a serious step, and it would require us to consult very closely with our colleagues on the six-party talks for what kind of response we should make."
He said a nuclear test "would be something where the North Koreans would be defying not only us, but our partners in the six-party talks, and action would ... have to be taken."
Shinzo Abe, secretary-general of Japan's ruling Liberal Democratic Party, said Tokyo would take the issue to the United Nations. "It is unthinkable not to impose any sanctions in case of a nuclear test," he said.
Discussions involving the two Koreas, the United States, China, Japan and Russia have been stalled since June after three inconclusive rounds. North Korea refused to participate in the fourth set of talks, originally scheduled for September 2004.
Washington's top envoy in that dispute, U.S. Assistant Secretary of State Christopher Hill, met Monday with South Korean officials.
"We are doing everything to get this six-party process going, and we really want to, but that does not mean we are not going to look eventually at other options," Hill told South Korean Foreign Minister Ban Ki-moon.
Talks between the two Koreas broke off in July after mass defections to South Korea from the North that Pyongyang labeled kidnappings.
With North Korea's chief delegate, Kim Man Gil, saying the bilateral discussions were vital to regenerate ties, Rhee made several suggestions for improving relations. North Korea wanted to talk about food aid and fertilizer for its spring planting season; Rhee said the size of such aid needs further consultations.
State Department spokesman Richard Boucher offered support for South Korean efforts to bring North Korea back to negotiations. He added, however, that United States believes "humanitarian assistance to the people of North Korea, including food or helping them grow food, shouldn't be conditioned or negotiated as part of the six-party talks."
Rhee proposed resuming Cabinet-level discussions next month, arranging more reunions in August for families separated for more than half a century, conducting a trial run of cross-border railways and sending a government delegation next month to the North's capital to participate in a celebration marking the fifth anniversary of a historic inter-Korean summit accord.
U.S. officials said last week that spy satellites looking at the North's northeastern Kilju spotted the digging of a tunnel and the construction of a reviewing stand -- possible indications of an upcoming test.
But Deputy Foreign Minister Song Min-soon played down the prospects of a nuclear test.
"The reports that are coming out are artificial and groundless that have no specific evidence to back them up," Song told South Korea's Yonhap news agency.
The latest nuclear standoff with North Korea was sparked in late 2002 after U.S. officials accused the North of running a secret uranium enrichment program.