Perhaps the most closely watched reaction came from China, North Korea's main ally and the source of crucial economic and political support to the regime in Pyongyang.
The Chinese Foreign Ministry said it "resolutely opposes" the North's latest test, which it noted had taken place "despite the international community's widespread opposition."
Beijing summoned the North Korean ambassador to China over its "dissatisfaction" with the test, the ministry said.
It said it "strongly" urged North Korean officials to "abide by their promise to denuclearize and take no further action that will worsen the situation."
The real question, though, is whether Beijing will support significantly tougher measures against its smaller neighbor following the test, something it has refrained from doing in the past.
"The Chinese don't like the idea of international sanctions and coercing other countries," Chinoy said. "They still have a strategic interest in maintaining a viable separate North Korea as a buffer against a pro-U.S. South Korea, and that has only become more important as tensions between the U.S. and China have increased."
Recent opinion articles published in the state-run Chinese newspaper Global Times suggested Beijing's patience with North Korea may be wearing thin and raised the prospect of reducing support to Pyongyang.
But with fears in Beijing of what a possible collapse of the North Korean regime could bring, strong measures appear unlikely for the time being.
"I think the key with China right now is that they are necessary to a solution, but we can't expect them to solve the problem for us," said Philip Yun, executive director of the Ploughshares Fund, a U.S.-based foundation that seeks to stop the spread of nuclear weapons.
Indications that the test had taken place first emerged when U.S. seismologists reported a disturbance Tuesday morning in North Korea centered near the site of the secretive regime's two previous atomic blasts.
The area around the epicenter of the tremor in northeastern North Korea has little or no history of earthquakes or natural seismic hazards, according to U.S. Geological Survey maps.
The disturbance reported Tuesday had a magnitude of 5.1 -- upgraded from an initial estimate of 4.9 -- and took place at a depth of about one kilometer, the USGS said.
Kim Min-seok, a spokesman for the South Korean Defense Ministry, said the magnitude of the "artificial tremor" suggested the size of the blast could be in the order of 6 to 7 kilotons, more powerful than the North's two prior nuclear tests.
That calculation, though, was based on the USGS's initial estimate of a 4.9-magnitude seismic disturbance, he said. A 5.1-magnitude tremor could indicate a 10-kiloton explosion.
News breaks amid key dates in Northeast Asia
The test took place at a time when several East Asian countries, including China, North Korea's major ally, are observing public holidays for the Lunar New Year, which began Sunday.
It also comes ahead of significant dates in both North and South Korea.
On Saturday, North Koreans will celebrate the birthday of Kim Jong Il, the former North Korean leader who died in December 2011 after 17 years in power and was succeeded by his son, Kim Jong Un.
And on February 25, the South Korea president-elect, Park Geun-hye, will take office. She had campaigned on a pledge to seek increased dialogue with the North, but Pyongyang's recent moves have left her little room for maneuver.