US President Barack Obama spoke to South Korean President Park Geun-Hye and to Prime Minister Shinzo Abe of Japan to forge a response to "North Korea's latest reckless behavior," it said.
Obama also reaffirmed the "unshakable US commitment" to the security of South Korea and Japan, it added.
South Korean Defense Minister Han Min-koo said Seoul was considering a variety of punitive measures to be taken against its northern neighbor.
The country was restarting border propaganda broadcasts that it halted after an agreement with Pyongyang in late August on a package of measures for easing tensions.
The Japan Times quoted a senior official in Tokyo as saying that a North Korea armed with a new nuclear weapon would expose Japan to its “biggest threat.”
“The North seems to be highly unpredictable now. That’s scary,” the unnamed official told the paper.
This is while on Wednesday the White House expressed doubt over claims by Pyongyang regarding the successful testing of a hydrogen bomb.
"The initial analysis that has been conducted ... is not consistent with North Korea's claim of a successful hydrogen bomb test," White House spokesman Josh Earnest said.
The qualms on the North Korea’s testing of a hydrogen bomb were generated after seismic activity was detected in the country, whose waves were seen as smaller than detonation of a true thermonuclear weapon.
The North Koreans were said to have only increased the yield of a normal nuclear device.
"Judging from the measurements, it probably falls short of being a hydrogen bomb although it (North Korea) claims it's a hydrogen bomb," Lee Cheol-woo, a member of the intelligence committee of the South Korean National Assembly, told reporters in Seoul.
Pyongyang’s move in conducting a new atomic test is considered by some as a means to gain the upper hand in its future negotiations with world powers.
North Korea responded to the condemnations by vowing to continue its nuclear program as deterrence against potential aggression from the US.
The country is under four rounds of UN sanctions over launching missiles considered by the US and South Korea as ballistic and aimed at delivering nuclear warheads
Pyongyang says its numerous missile tests seek to boost its defense capabilities in the face of enemy threats.
Reacting to the criticism, North Korea's state news agency stressed in a statement that the country will act as a responsible nuclear state, and will use its nuclear armament only to defend its sovereignty, Press TV reported.
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