Nuclear envoys from South Korea, Japan and the United States agreed Wednesday to step up pressure -- including possible sanctions -- on North Korea to abandon nuclear weapons, while keeping diplomatic options "open."

Nuclear envoys from South Korea, Japan and the United States agreed Wednesday to step up pressure -- including possible sanctions -- on North Korea to abandon nuclear weapons, while keeping diplomatic options "open."

Speaking to reporters after trilateral talks in Seoul, the three officials also highlighted the "seriousness" of the progress the North has been making with its expanding nuclear and ballistic missile programs.

"We agreed on the importance of enhancing pressure and sanctions on North Korea even as we keep all our diplomatic options on the table and open," the US special representative for North Korea policy, Sung Kim, told reporters.

The talks between Kim, South Korea's Hwang Joon-Kook and Japan's Junichi Ihara lasted around three hours and included discussions on the possibility of reviving long-stalled six-party talks aimed at pushing Pyongyang towards denuclearization.

North Korea has hailed the recent "historic" test of a submarine-launched ballistic missile (SLBM).

Hwang said the three envoys had all pointed to the "seriousness of the advancement of the North's nuclear capability", while Sung Kim said the SLBM test -- whether genuine or not -- was also unnerving.

"It is of great concern to us that the North Koreans are continuing to pursue such capabilities," Kim said.

"I think their intention is clear, and we should be concerned regardless of the stage of their development," he added.