Foreign Minister Yun Byung-se on Thursday warned North Korea of “unimaginable consequences” amid threats of a fresh nuclear test, calling on its leadership to choose between coexistence or isolation.

Concerns have grown after North Korea repeatedly threatened to conduct a “new type of nuclear test” since late last month to boost its nuclear deterrence in the face of ongoing South Korea-U.S. military drills and the U.N. Security Council’s condemnation of its recent test of ballistic missiles.

Officials say there are no signs of an impending underground detonation. But Seoul, Washington, Beijing and other world powers are ramping up diplomatic efforts to preclude any further provocations from the North.

“North Korea has recently spoken of ‘unimaginable next steps’ and a ‘new type of nuclear test.’ It will be North Korea that faces unimaginable consequences if it undertakes an additional nuclear test,” the top diplomat told a conference hosted by a think tank affiliated with the ruling Saenuri Party in Seoul.

In the event of a fourth atomic blast, Pyongyang’s relations with the international community will take on a “completely different” aspect,” he warned.

“North Korea will have to make a serious judgment about what the nuclear test would mean to its regime at a time when the international community including China is tightly united against North Korean nuclear weapons,” Yun added.

“The North Korean leadership must bear in mind that its parallel pursuit of nuclear and economic development will never succeed and must make the right strategic choice between coexistence and isolation.”

In a report to the National Assembly, the Foreign Ministry said it would push for an additional set of “strong” sanctions at the U.N. Security Council.

The council’s resolution imposed after the communist state’s third nuclear experiment in February 2013 contains a “trigger” clause, which forces an automatic gathering, paving the way for a quicker resolution and sanctions.

The ministry also said the North’s recent missile tests and live-fire drills were “premeditated provocations” aimed at covering up instability and reinforcing unity at home while testing the South’s defense readiness, protesting international opposition to its nuclear programs and boasting its ballistic missile technology abroad.

With rising concerns, Seoul is stepping up coordination with the members of the six-party talks on the denuclearization of the North.

On Tuesday, the chief negotiators of South Korea, the U.S. and Japan on North Korea’s nuclear weapons program agreed to expand cooperation to dissuade the communist state from further provocations.

Hwang Joon-kook, Seoul’s new special representative for Korean peninsular peace and security affairs and chief envoy to the six-party talks, plans to travel to China on Friday to meet with officials including Wu Dawei, China’s top nuclear envoy and special representative for Korean Peninsular affairs. They are expected to discuss the current situation on the peninsula and ways to curb the North’s nuclear brinkmanship and restart the six-nation forum.

Yun, for his part, phoned his Chinese counterpart Wang Yi late Wednesday and had “candid, in-depth” discussions on Pyongyang’s nuclear program and other issues of common interest, the ministry said.

During the one-hour conversation, the two diplomats agreed to “make maximum diplomatic efforts” to prevent additional provocations and tension on the peninsula and maintain peace and stability in the region, it added.

By Shin Hyon-hee (heeshin@heraldcorp.com)