Barack Obama, the US president, has condemned a North Korean underground nuclear missile test as "reckless", as the UN Security Council held an emergency meeting on the issue.
Obama said the US "and the international community must take action in response" to Monday's explosion, which was said to be 20 times more powerful than previous tests by Pyongyang.
"North Korea's nuclear and ballistic missile programmes pose a grave threat to the peace and security of the world and I strongly condemn their reckless action," Obama said at the White House.
Obama called the test "a blunt violation of international law" and said Pyongyang had reneged on its commitment to abandon its nuclear ambitions.
The UN Security Council unanimously condemned the nuclear test, saying it was a "clear violation" of a resolution passed in 2006 after Pyongyang's first atomic test.
"The members of the Security Council have decided to start work immediately on a Security Council resolution on this matter," said the organisation in a statement after Monday's meeting.
The UN talks were preceded by a "P5 + 2" session between the five permanent members of the Security Council - Britain, China, France, Russia and the US - plus Japan and South Korea.
Ban Ki-Moon, the UN secretary general, also joined widespread international criticism of the test, saying he was "deeply disturbed" by reports the test had taken place.
|Ban was "deeply disturbed" by the North
Korean missile test [AFP]
"They should have come to the dialogue table and resolved all the issues through peaceful means," he said.
John Bolton, a former US ambassador to the UN during the Bush administration told Al Jazeera the US needed respond to the test using "strong measures".
"I think we should conclude from this that North Korea is not going to be talked out of its nuclear programme".
"I would recommend economic sanctions along the lines of those imposed on Iraq after the invasion of Kuwait in 1990...I believe this is a case that calls for North Korea to be expelled from the United Nations for persistent violations of its charter".
Bolton also said the US should "return North Korea to the list of state sponsors of terrorism".
China, Japan, Russia, South Korea and the US have been negotiating since 2003 to persuade the North to abandon its nuclear weapons work in exchange for energy and security guarantees.
|The test site is believed to be near the northeastern town of Kilchu
The six-party talks led to a 2007 agreement under which the North said it would dismantle its nuclear facilities, but the deal stalled during disagreements over verification.
In April, North Korea said it launched a satellite into orbit, although the the US has said it believes the launch was a cover for a test of the North's long-range missile technology.
The launch triggered condemnation from the Security Council, in turn provoking an angry reaction from the North, which said it was pulling out of nuclear disarmament talks and restarting its weapons programme.
Russia's defence ministry said an atomic explosion at had taken place in northeastern North Korea, 0954am (0054GMT) and estimated the blast's to be up to 20 kilotonnes, about the same as the US bombs used against the Japanese cities of Nagasaki and Hiroshima at the end of World War Two.
The US Geological Survey (USGS) earlier said it had detected a magnitude 4.7 tremor near the town of Kilchu about 375km northeast of Pyongyang, the capital, and close to where North Korea conducted its first nuclear test in October 2006, which was estimated to be about one kilotonne.