South Korean Foreign Minister Yun Byung-se and U.S. Secretary of State John Kerry agreed that the North's fourth nuclear test is an unacceptable challenge to the international community, Seoul's Foreign Ministry said Thursday.

The two top diplomats spoke by phone to discuss responses to the North's claims of an successful H-bomb test, denouncing the detonation as a threat to peace and security on the Korean Peninsula and around the world, the ministry said in a statement.

They also agreed to respond sternly to any provocations, it said.

Yun underscored the importance of a "swift and strong" U.N. Security Council resolution to punish the North and pledged to work with the United States and other countries for various diplomatic responses to the North's provocation, the ministry said.

Kerry said the North's test is an unacceptable and clear violation of U.N. Security Council resolutions and the U.S. will deal sternly with the North's move while working closely with the U.N. and other members of the international community, it said.

On Wednesday, Yun also spoke by phone with Japanese Foreign Minister Fumio Kishida. Officials said they were working to set up similar phone calls with Chinese Foreign Minister Wang Yi and Russian Foreign Minister Sergei Lavrov.

Kerry also later issued a statement condemning the North's test as "highly provocative act" that poses "a grave threat to international peace and security and blatantly violates multiple U.N. Security Council resolutions."

"As I am reiterating today in conversations with my counterparts overseas, the U.S. is committed to defending the American people and honoring our security commitments to our allies in the region," he said.

"We do not and will not accept North Korea as a nuclear armed state, and actions such as this latest test only strengthen our resolve. We will continue to work closely with our partners on the U.N. Security Council and in the six-party talks to take appropriate action," he said.

Kerry also urged Pyongyang to end provocations and choose a better path.

"North Korea will only achieve the security and development it claims to seek by living up to its international obligations and commitments," he said. (Yonhap)