A hydrogen bomb is more powerful than plutonium weapons, which is what North Korea used in its three previous underground nuclear tests.
"If there's no invasion on our sovereignty we will not use nuclear weapon," the North Korean state news agency said. "This H-bomb test brings us to a higher level of nuclear power."
A senior U.S. administration told CNN it could take days to obtain the scientific data to determine whether this was a successful test.
The South Korean defense ministry said it too could not immediately confirm the test's success, but the country's foreign ministry hastily convened an emergency meeting. Officials in Japan were also holding discussions.
The test took place at 10 a.m. local time, the regime said in a televised statement.
The seismic event, which measured the event at a magnitude of 5.1, occurred 19 kilometers (12 miles) east-northeast of Sungjibaegam, the United States Geological Survey said. Read more on edition.cnn.