The White House has cast doubt on North Korea's claim to have conducted its first "successful" test of a miniature hydrogen bomb.
Pyongyang said the detonation was a "historic" event, but White House press secretary Josh Earnest said initial analysis was "not consistent" with the claim.
The UN Security Council has strongly condemned North Korea's purported Wednesday morning test and pledged to pursue new sanctions.
North Korean state television trumpeted the news, announcing: "With the perfect success of our historic H-bomb, we have joined the ranks of advanced nuclear states."
The announcer said it was a "miniaturised" device that was tested.
Crowds watched the news on a screen in front of Pyongyang Railway Station.
"I cannot think of other words than 'great'," said one resident of the capital city.
"We must keep putting our country in the first place unconditionally in the future."
The country's leader, Kim Jong-Un, scribbled on the detonation order: "Let's begin the year of 2016 with the thrilling sound of our first hydrogen bomb explosion.
"So that the whole world will look up to our socialist, nuclear-armed republic."
The head of the nuclear test ban treaty organisation, CTBTO, said the magnitude of the seismic event in North Korea appears to have been less than a similar one three years ago.
Pyongyang has conducted three nuclear tests - in 2006, 2009 and 2013 - with the last also measuring 5.1 on the USGS scale.
South Korea's meteorological agency said it detected an artificial earthquake 30 miles (49km) from the Punggye-ri site where the North has conducted previous tests.
The US Geological Survey said the quake measured 5.1 on the USGS scale.
If verified, the test would mark a major, and alarming, step forward in the isolated country's nuclear development, as hydrogen bombs are generally more powerful than nuclear bombs.
A hydrogen (thermonuclear) device, uses fusion in a chain reaction that causes a more powerful explosion than the fission blast generated by uranium or plutonium.
North Korea is thought to possess a handful of crude nuclear weapons and has long pushed for an arsenal of warheads that can be mounted on a ballistic missile.
The International Atomic Energy Agency and NATO Secretary General Jens Stoltenberg also condemned the purported test.
South Korea's defence ministry said its armed forces would step up their monitoring of the North, while Japanese Prime Minister Shinzo Abe said it was a "serious threat".
Jason Strother is a journalist based in the South Korean capital, Seoul. He told Newstalk Breakfast the public there has largely ignored the claim.
