North Korea's strictly unauthorized nuclear test has drawn sharp criticism from around the world, with even China joining the outcry against its ally.
The US, North Korea's so-called "arch enemy," led the condemnation. In a statement, President Barack Obama called North Korea's third successful nuclear test a "threat to US national security," and hinted at action by the international community.
Obama also called the underground explosion — registered by US seismologists as a 4.9-magnitude earthquake — a "highly provocative act" that "undermines regional stability."
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On the other side of the border, South Korea condemned the test as an "unacceptable threat" and a "head-on challenge to the international community."
"North Korea won't be able to avoid grave responsibility," the government warned in a statement, after an emergency meeting of the National Security Council.
Meanwhile China, North Korea's most powerful ally, summoned the country's ambassador to lodge a formal protest.
The Chinese government is "strongly dissatisfied" and "firmly opposed to this act," the Foreign Ministry said in a statement, urging Pyongyang to refrain from further provocation and return to talks.
UN Secretary-General Ban Ki-moon also condemned the test, which he said was "a clear and grave violation of the relevant Security Council resolutions." He called it "deplorable" that North Korea had chosen to proceed despite international calls to halt.
Other reactions from around the world:
• Japanese Prime Minister Shinzo Abe called the test a "grave threat" to Japan's safety. The government convened a national security meeting and is contemplating imposing unilateral sanctions on North Korea, according to local media.
• NATO said the test was an "irresponsible act" and a "grave threat to international and regional peace, security and stability."
• Russia's Foreign Minister Sergei Lavrov urged North Korea to "abandon its nuclear arms program" and resume talks with other countries.
• William Hague, Britain's foreign secretary, called for a "robust response" from the UN Security Council.
• French President François Hollande said he would back "firm action" by the UN.
• Iran's Foreign Ministry said all nuclear weapons should be destroyed, but commented that "all countries should have the right to make use of nuclear activities for peaceful purposes."
The UN Security Council is due to hold an emergency meeting in New York at 9 a.m. to discuss its response.