Nonetheless the claim sparked concern internationally, prompting the United Nations Security Council (UNSC) to convene an emergency meeting to be held Wednesday to discuss the matter.
A statement issued by the council after emergency closed-door consultations on Wednesday called the test "a clear violation" of council resolutions, adding "therefore a clear threat to international peace and security continues to exist."
The council said it had previously expressed determination to take "further significant measures" in the event of another North Korean test and would begin work immediately on a new sanctions resolution in light of "the gravity of this violation."
United Nations Secretary-General Ban Ki-moon also condemned North Korea's announcement, calling it "profoundly destabilizing for regional security."
Samantha Power, the U.S. ambassador to the United Nations, in a statement issued after the council's emergency session, called on the UNSC to hold North Korea accountable "by imposing a tough, comprehensive and credible package of new sanctions" and "steadily increasing pressure" in response to that country's announced nuclear test.
In the days and weeks ahead, nuclear experts will be hunting for airborne radioactive particles that could shed light on North Korea's assertion that it tested a hydrogen bomb.
The detection of airborne radioactive particles that will give clues as to the type of device that was set off and whether it was a hydrogen bomb, which is more powerful than an atomic bomb and would mark a technological advance for North Korea.
Another possibility is that it was not a nuclear device at all but a conventional high-yield explosive.
Following the North's last nuclear test, in 2013, it was 55 days before radioactive xenon gas was detected at a monitoring station in Japan, located about 600 milesfrom the test site, which pointed to a nuclear blast by Pyongyang.
News of the purported test was relayed by an anchor on North Korean state TV. In the report she said the North had tested a "miniaturized" hydrogen bomb, elevating the country's "nuclear might to the next level" and providing it with a weapon to defend against the United States and its other enemies.
If confirmed, it would mark the fourth time the isolated state has exploded a nuclear device. But officials in several countries expressed doubt.
White House Press Secretary Josh Earnest said Wednesday that "the initial analysis that’s been conducted of the events that were reported overnight is not consistent with North Korean claims of a successful hydrogen bomb test."
"There’s nothing that has occurred in the last 24 hours that’s caused the United States government to change our assessment of North Korea’s technical and military capabilities," Earnest added.
South Korea held an emergency meeting on Wednesday and later said it would take all possible measures to respond to its long-time foe's actions.
“Our government strongly condemns North Korea ignoring repeated warnings from us and the international community and pushing ahead with the fourth nuclear test, which clearly violated the U.N. resolutions,” Cho Tae-yong, a senior security official at the South Korean presidential office, said.
The test was unexpected in part because North Korea's last nuclear test was nearly three years ago and Kim Jong Un did not mention nuclear weapons in his annual New Year's speech. Some outside analysts had speculated Kim was worried about deteriorating ties with China, the North's last major ally, which has shown greater frustration at provocations and a possible willingness to allow stronger U.N. sanctions.
Chinese Foreign Ministry spokeswoman Hua Chunying told reporters that Beijing “firmly opposes” Pyongyang's purported test and is monitoring the environment on its border with North Korea near the test site.
Japanese Prime Minister Shinzo Abe called North Korea’s announcement a threat to his nation's safety. “We absolutely cannot allow this, and condemn it strongly,” he said.
International Atomic Energy Agency chief Yukiya Amano said in a statement that the North's nuclear test “if confirmed, is in clear violation of U.N. Security Council resolutions and is deeply regrettable.”
The United States Geological Survey (USGS) detected the quake that South Korea said was 30 miles from the Punggye-ri site where the North has conducted nuclear tests in the past. While the USGS put the depth of the earthquake at 6.2 miles, the South Korean agency said it was near the surface. The earthquake was detected just after 10 a.m. Seoul time.
Pyongyang has been under U.N. sanctions due to its nuclear weapons program since it first tested an atomic device in 2006. One Western diplomat told Reuters that if the latest North Korean nuclear test was confirmed, council members would seek to expand existing U.N. sanctions against Pyongyang.
Al Jazeera and wire services. Jamie McIntyre contributed to this report from Washington D.C.