North Korea drew threats of further sanctions from the UN after claiming to have tested a nuclear bomb on Wednesday
The underground explosion triggered US quake monitors and angered China, putting pressure on Beijing to rein in its neighbour and ally. The United States and weapons experts voiced doubts the device was as advanced as the isolated nation claimed.
The test also alarmed Japan, whose leader told US president Barack Obama in a phone call that a firm global response was needed, Kyodo news agency said.
The UN Security Council said it would work immediately on significant new measures, a threat diplomats said could mean an expansion of sanctions against Pyongyang, although major powers might baulk at an all-out economic offensive.
North Korea has been under Security Council sanctions since it first tested an atomic device in 2006.
After a nuclear test in 2013, the Security Council took about three weeks to agree a resolution that tightened financial restrictions and cracked down on Pyongyang’s attempts to ship and receive banned cargo.
In the United States, Republican presidential candidates seized on the test to accuse Obama of running a “feckless” foreign policy that enabled North Korea to bolster its nuclear arms capabilities.
US congressional sources said Republican leaders of the US House of Representatives were considering a vote as soon as next week to impose stiffer punishment on foreign companies doing business with Pyongyang.
North Korea has a long history of voicing bellicose rhetoric against the United States and its Asian allies without acting on it, but Pyongyang’s assertion that it had tested a hydrogen device, much more powerful than an atomic bomb, came as a surprise.
North Korea also said it was capable of miniaturising the H-bomb, in theory allowing it to be placed on a missile and potentially posing a new threat to the US West Coast, South Korea and Japan.
The US State Department confirmed North Korea had conducted a nuclear test but the Obama administration disputed the hydrogen bomb claim.
"The initial analysis is not consistent with the claim the regime has made of a successful hydrogen bomb test," White House spokesman Josh Earnest told reporters.
He said any nuclear test would be a “flagrant violation” of Security Council resolutions.
"It is a grave security threat to damage the peace and safety of the region and international society," Japanese broadcaster NHK quoted Prime Minister Shinzo Abe as saying after he spoke to Obama.
Wednesday's nuclear test took place two days ahead of what is believed to be North Korean leader Kim Jong Un's birthday.