Negotiators and representatives from five of the six countries involved in the stalled talks on ending North Korea’s nuclear arms program held an informal meeting in Tokyo on Thursday in the absence of Pyongyang, a Japanese diplomatic source said, in a move aimed at stepping up pressure against the North.

Representing Japan, the United States, South Korea, Russia and China, they discussed applying stronger sanctions against Pyongyang to push it to take more concrete steps to denuclearize, the source said. North Korea is also part of the six-party framework.

But the five nations failed to agree on tightened sanctions due to opposition by China and Russia, the source said.

Among those who attended the meeting was Shigeki Takizaki, deputy director general of the Foreign Ministry’s Asian and Oceanian Affairs Bureau, and Sydney Seiler, special U.S. envoy for the six-party talks on North Korea’s denuclearization, the source said.

The meeting came a day after top negotiators of the United States, Japan, and South Korea met in Seoul and agreed to increase “pressures and sanctions” on North Korea to dissuade the North from military provocations.

Deputy chief negotiators of Japan, South Korea and the United States also held a separate meeting to discuss the current status of North Korea’s nuclear weapons program and the kind of sanctions that could be imposed on the North, the source said.

The representatives who attended the informal meetings were in Tokyo to attend the Northeast Asia Cooperation Dialogue, a U.S. research institute-sponsored conference.

Japan initially sought to hold an informal meeting of countries involved in the six-party framework on the occasion of the conference but the plan did not push through after Pyongyang declined to send representatives.

“We very much regret that the DPRK declined our invitation to attend this meeting with no explanation and we will continue to engage with the DPRK in the hopes that it will attend the next meeting,” Susan Shirk, former U.S. deputy assistant secretary of state, said at the outset of the one-day dialogue.

DPRK is the acronym for North Korea’s official name, the Democratic People’s Republic of Korea.

North Korean representatives appear to have skipped the meeting amid heightened tensions over its recent claim to have fired a submarine-launched ballistic missile and developed technology allowing the North to miniaturize nuclear warheads, according to another diplomatic source.

The dialogue, organized by the University of California Institute on Global Conflict and Cooperation, is a forum for government officials, military officers and academics from the six countries to discuss regional security and cooperation.

Shirk said the dialogue enables participants to “have candid, off-the-record discussions to help improve understanding, reduce mistrust and come up with creative ideas for policies, particularly new forms of cooperation among the six countries.”

The latest conference was attended by representatives from China, Japan, Russia, South Korea and the United States. The conference is part of what is dubbed “second-track” diplomacy that also involves civilian experts and is held almost annually in different venues.

Speaking at the conference, Yoshiji Nogami, president of the Japan Institute of International Affairs, expressed hope for “constructive and intensive” discussions on the changing security landscape in Northeast Asia in relation to unpredictable situations involving North Korea.

The latest dialogue was held in cooperation with the Tokyo-based institute.

When the conference was held in Tokyo in 2006, then North Korean Vice Foreign Minister Kim Kye Gwan, who was chief negotiator at the six-party talks, attended the meeting and set the stage for the resumption of the talks.

The six-party talks aimed at ending North Korea’s nuclear ambitions have been deadlocked since late 2008.

Japan wants to boost its coordination with the other participants in the six-party framework as it tries to make progress on North Korea’s reinvestigation into the fate of Japanese nationals abducted by Pyongyang decades ago.