The 5.3 magnitude seismic event on Friday indicated the blast was 10 kilotons in yield. If so, it would be the largest ever conducted by the North. The quake was detected near North Korea's Punggye-ri nuclear test site as the country celebrates Foundation Day, marking the anniversary of the founding of the nation in 1948.

"The artificial... earthquake in the North... is likely a nuclear test," a South Korean meteorological agency official said, according to the Yonhap news agency.

South Korean President Park Geun-hye said North Korea has violated UN Security Council resolutions, in a direct challenge to the international community. Kim Jong Un, he said, was displaying "maniacal recklessness."

Japan's Prime Minister Shinzo Abe said on Friday the suspected nuclear test "could not be tolerated" and that Japan would protest strongly to Pyongyang if confirmed. Foreign Minister Fumio Kishida vowed that if a nuclear test is confirmed, Japan will "lodge a strong protest against North Korea... and immediately respond at the UN Security Council."

Japan's Chief Cabinet Secretary Yoshihide Suga told an emergency press conference that the government was acting swiftly to gather information.

"There is a high possibility that it was a nuclear test, given the location and the magnitude of the quake," another unidentified government official told Yonhap. "We think that there is a possibility that this quake occurred because North Korea carried out a nuclear test," he said.

Pyongyang test-fired three missiles Monday as world powers gathered for a G20 meeting in China, with leader Kim Jong-Un hailing the tests as "perfect", and US President Barack Obama warning it would only up the pressure.

(DW)