North Korea's claim that it carried out a successful hydrogen bomb test Wednesday drew swift condemnation from friends and foes alike.
China said it "firmly opposes" its neighbour's actions while NATO condemned the test as a threat to regional and international security.
Several governments promised a firm response as tensions soared, with many calling for further action by the United Nations against North Korea, which is already subject to an array of international sanctions.
The UN Security Council was to hold an emergency session later Wednesday.
China, North Korea's most important diplomatic and economic partner, took a more nuanced stance than others, saying it "firmly opposes" the test and would summon Pyongyang's ambassador for "solemn representations".
It added that dialogue was the "only practical way to resolve the relevant issue".
Beijing is a key provider of aid and trade to the Democratic People's Republic of Korea (DPRK) but relations have become more strained in recent years, in part because of Pyongyang's persistence with its nuclear programme in the face of international condemnation.
North Korean leader Kim Jong-Un has yet to visit Beijing since coming to power following the death of his father four years ago.
"We strongly urge the DPRK side to remain committed to its denuclearisation commitment, and stop taking any actions that would make the situation worse," foreign ministry spokeswoman Hua Chunying told a regular briefing, using the North's official name.
South Korean President Park Geun-Hye described the test as a "grave provocation" at an emergency meeting of the country's National Security Council.
"The test is not only a grave provocation to our national security but also a threat to our future... and a strong challenge to international peace and stability," she said, calling for strong sanctions on Pyongyang.'Grave concern'
Japanese Prime Minister Shinzo Abe slammed it as "a serious threat to the safety of our nation".
"This clearly violates UN Security Council resolutions and is a grave challenge against international efforts for non-proliferation," he said.
In Washington the White House would not confirm the test, but vowed to "respond appropriately to any and all North Korean provocations".
NATO head Jens Stoltenberg condemned the test and said North Korea should abandon nuclear weapons.
"The nuclear weapons test announced by North Korea undermines regional and international security, and is in clear breach of UN Security Council resolutions," Stoltenberg said in a statement.
The foreign ministry of Russia, a permanent Security Council member, denounced the test as a "flagrant violation of international law and existing UN Security Council resolutions".
"Such actions are fraught with the possibility of aggravating the situation on the Korean peninsula, which already has a very high potential for military and political confrontation," it said.
In a phone call with South Korea's top diplomat Hwang Joon-kook, who leads negotations on North Korea's nuclear disarmament, Russia's deputy foreign minister Igor Morgulov expressed Moscow's "grave concern".
Other veto-wielding Security Council members Britain and France also joined in the chorus of condemnation.
Speaking in Beijing, Britain's Foreign Secretary Philip Hammond said the test was "a grave breach of UN Security Council resolutions and a provocation".
Paris labelled the move an "unacceptable violation" of UN resolutions and called for a strong reaction from the international community.'Rogue state'
Australian Foreign Minister Julie Bishop said her country "condemns in the strongest possible terms" the test, which "confirms North Korea's status as a rogue state and a continuing threat to international peace and security", adding that Canberra would express its concerns to Pyongyang directly and call for stronger UN sanctions.
The test, which came just two days before leader Kim's birthday, was initially detected by international seismology monitors as a 5.1-magnitude tremor next to the North's main Punggye-ri nuclear test site in the northeast.
Last month Kim suggested Pyongyang had already developed a hydrogen bomb.
The claim was questioned by international experts and there was continued scepticism over Wednesday's test announcement.
Whether an H-bomb or not, it was North Korea's fourth nuclear test and marked a striking act of defiance in the face of warnings that Pyongyang would pay a steep price if it continued pursuing its atomic weapons programme.
The three previous tests in 2006, 2009 and 2013 triggered waves of UN sanctions. Their failure to prevent a fourth detonation will see calls for more drastic Security Council action this time around.