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Britain has urged South Korea to show restraint after Seoul resumed propaganda broadcasts in retaliation for North Korea's nuclear bomb test.
During a trip to Japan, Foreign Secretary Philip Hammond said: "We urge South Korea to exercise restraint. It is simply rising to the bait."
South Korea unleashed a high-decibel broadcasts across the border earlier on Friday.
The broadcasts, considered an insult by the North, led to an exchange of artillery fire the last time they were used.
The UN Security Council met to discuss more sanctions on North Korea after it claimed to have denoted its first hydrogen bomb.
It said it violated UN resolutions and was destabilising to the region.
Doubt has been cast over whether it was a hydrogen bomb, but if true it could mark a major upgrade in the country's capabilities.
ITV News International Affairs Editor Rageh Omaar reports:
The White House said initial analysis of North Korea's reported nuclear test is not consistent with claims of a successful hydrogen bomb.
The White said they are "continuing the work necessary to learn more" about the reported nuclear test.
It added any kind of nuclear test is "provocative and a flagrant violation" of UN Security Council resolutions.
The White House said it had a "rock solid commitment" to the safety of its allies Japan and South Korea.
The White House said there have been conversations with Chinese officials over the test and "this is an issue on which we do agree with China and that agreement is significant".
It said the US will be "collecting addition evidence" and analysis to learn more about the test.
Presidential candidate Hillary Clinton has "strongly condemned" North Korea's apparent nuclear test.
She said: "If verified, this is a provocative and dangerous act, and North Korea must have no doubt that we will take whatever steps are necessary to defend ourselves and our treaty allies, South Korea and Japan."
North Korea could face new sanctions over a reported hydrogen nuclear device test, a top US Democrat has warned.
Senator Ben Cardin, who sits on the US Senate Foreign Relations Committee, said he will pursue legislation to impose fresh sanctions on the state.
Mr Cardin said the United States and other nations "must respond resolutely" to the test.
"In particular, I intend to work with my colleagues in the Senate on legislation to impose additional sanctions on North Korea and would also urge additional sanctions by the United Nations Security Council," he said in a statement.
North Korea's hydrogen bomb test claims "look like a provocation" but "we don't know the facts yet", the US House Speaker said.
Paul Ryan said: "We don't know that he facts yet. This looks like a provocation."
US politician Adam Schiff added it will likely take several days for the intelligence community to evaluate the validity of North Korea's hydrogen bomb test claims.
Mr Schiff, part of the US House of Representatives Intelligence Committee, was sceptical of Pyongyang's announcement.
He said in a statement: "Given North Korea's often boastful claims in the past, its assertion that it has mastered the science necessary for a thermonuclear explosion cannot be accepted uncritically."
North Korea's nuclear test is "deeply troubling" and "profoundly destabilising for regional security", United Nations Secretary-General said.
Ban Ki-moon said: "This test once again violates numerous Security Council resolutions despite the united call by the international community to cease such activities.
He added it is a "grave contravention of the international norm against nuclear testing."
He demanded that North Korea cease any further nuclear activities and "meet its obligations for verifiable denuclearisation".
It will be possible to coordinate real international action if the Chinese have decided that the North Korean detonation is a "provocation too far", a former British ambassador to North Korea has said.
John Everard, 59, was the British ambassador to the hermit kingdom between 2006 and 2008 - during the period of North Korea's first nuclear test.
China - North Korea's most powerful ally - has said that it will work with the international community to denuclearise the Korean peninsula, Mr Everard said.
Mr Everard said the Chinese will have to show "at least part of their hand" at the United Nations Security Council meeting tonight.
However, he said the Chinese are worried that destabilising the Korean Peninsula could add to economic and social problems in its own regions.
Latest ITV News reports
A US military statement said the low-level flight over ally South Korea was in response to the North's 'recent provocative action'.
Tens of thousands of people danced and applauded in central Pyongyang, North Korea, in support of the country's reported H-bomb test.