Japanese and South Korean meteorological officials said an quake detected near the North's test site - measured by the U.S. Geological Survey (USGS) at magnitude 6.3 - was around 10 times more powerful than previous detonations.
North Korea says it has a hydrogen bomb with an explosive power that is adjustable up to hundreds of kilotons.
According to the USGS, the seismic activity - which the scientific agency has not identified as an natural disaster per se - occurred 24 kilometers east northeast of Sungjibaegam, North Korea.
South Korea immediately convened a national security council meeting after the tremor, which US seismologists say was recorded at a depth of six miles (10 km).
North Korea conducted two nuclear tests last year, the last almost a year ago, on the 9 September anniversary of the nation's founding.More news: Trump says Missouri Sen. McCaskill should back tax cuts or lose re-election
North Korea test fired two ICBMs, Hwasong 14, in July, resulting in a resolution adopted last month by the United Nations Security Council imposing more sanctions on Pyongyang. "We have to strongly protest it", Abe said.
"Today's test is a highly unsafe affront to the entire worldwide community".
"North Korea has a choice".
South Korea's spy agency recently told lawmakers that the North remains ready to conduct its sixth nuclear test any time.
Initial reports from the US Geological Survey put the tremor at 5.6 magnitude with a depth of 10km but this was later upgraded to 6.3 magnitude at 0km.More news: Caixin China General Manufacturing PMI at 51.6 for August
Hours earlier, Japanese Prime Minister Shinzo Abe spoke to President Trump on the phone about the "escalating" situation.
North Korea's claim that "this warhead is variable-yield and capable of specialized weapons effects implies a complex nuclear strategy".
Questions remain over whether it has successfully miniaturised its weapons, and whether it has a working H-bomb, but the official Korean Central News Agency said that leader Kim Jong-Un had inspected such a device at the Nuclear Weapons Institute.
The development comes amid heightened regional tension following launch of an intermediate-range missile by North Korea that flew over Japan.
North Korea's latest claims have not been independently verified.More news: Volkswagen introduce scrappage scheme worth up to £6000