The chief nuclear negotiators of South Korea, the U.S. and Japan agreed Wednesday to strengthen pressure and sanctions against North Korea to bring it to the dialogue table and keep it from further escalating nuclear threats.

Seoul’s top nuclear envoy Hwang Joon-kook and his U.S. and Japanese counterparts, Sung Kim and Junichi Ihara, held a trilateral meeting in Seoul over an array of North Korean issues including the human rights issue, after holding separate bilateral meetings the previous day.

The meeting came as Pyongyang heightened tensions by claiming to have conducted an underwater ejection test of a submarine-launched ballistic missile and to have already entered a technical phase to “miniaturize and diversify” nuclear bombs.

“We reaffirmed our commitment to continuing the closest possible trilateral coordination and consultation and also maintaining five-party unity including China and Russia,” Sung Kim told reporters after the meeting, referring to five members of the six-party denuclearization talks ― South Korea, the U.S., China, Japan and Russia.

“We agreed on the importance of enhancing pressure and sanctions on North Korea even as we keep all diplomatic options on the table and open,” he added, noting Pyongyang has rejected the international community’s “outreach” efforts, leaving it no choice but to raise pressure on the North.

High on the agenda for the tripartite meeting was the North’s pursuit of the SLBM, which would pose a tougher security challenge to South Korea and others as the SLBM provides capabilities to launch a retaliatory nuclear strike.

Seoul has referred the communist state to a sanctions committee of the U.N. Security Council, arguing that the underwater test of the SLBM constitutes a violation of UNSC resolutions that ban North Korea’s use of ballistic missile technology.

Hwang of South Korea said that at the trilateral meeting, all participants explored ways to better employ a mixture of pressure and dialogue to cope with “uncertainties and seriousness” of North Korea’s nuclear issue.

“Pyongyang has yet to respond to our offer of unconditional talks,” Hwang told reporters.

“As the nuclear threats escalate, the pressure from the international community will also increase, which will further deepen the North’s isolation. We urge the North to come out for dialogue with a sincere attitude.”

To ensure close coordination with China, Hwang and Kim will fly to Beijing on Thursday. They will hold talks with China’s top nuclear envoy Wu Dawei to discuss a range of North Korea-related issues including the resumption of the six-party talks that have been stalled since late 2008.

“I expect to have full consultations with Ambassador Wu Dawei tomorrow (Thursday) on how the Chinese may engage North Koreans to bring North Koreans to some credible and authentic negotiations,” said the U.S. negotiator Kim.

The flurry of diplomatic talks over North Korea’s nuclear program underscores the pressing need to resume dialogue with the North.

But skepticism remains high over any negotiations with the North since it identifies itself as a nuclear-armed state in its constitution and sticks to its so-called “Byungjin” policy line of simultaneously pursuing the development of nuclear arms and its economy.

By Song Sang-ho (sshluck@heraldcorp.com)