International Atomic Energy Agency (IAEA) Director Yukiya Amano told reporters in Seoul on Friday that the IAEA did not have the capacity to determine whether the North had tested a hydrogen bomb, as Pyongyang has claimed.

North Korea conducted its sixth nuclear test on September 3.

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Amano said this showed Pyongyang had made rapid progress on weapons development.

"[The] yield is much bigger than the previous test, and it means North Korea made very rapid progress," Amano said after a meeting with South Korean Foreign Minister Kang Kyung-wha. "Combined with other elements, this is a new threat and this is a global threat."

Tensions were already high after North Korea tested two intercontinental ballistic missiles in defiance of international pressure, but have increased since September 3, with a new round of sanctions against the North after a unanimous UN Security Council resolution.

"What is most important for now is for the international community to unite," Amano said.

A war of words between North Korean leader Kim and US President Donald Trump has also raised concerns in capitals around the globe.

South Korea warned on Thursday that the North may engage in more provocations as the anniversary of the founding of the North Korean communist party nears.

Members of the international community have urged both countries to resolve matters peacefully while boosting pressure on Pyongyang to curb its weapons programs.

A US State Department official said on Thursday that China was making progress in enforcing sanctions imposed on North Korea.

Read more: China orders North Korean businesses to close in line with latest UN sanctions

jbh/sms (AFP, Reuters)