The United States, South Korea and Japan said Thursday they would push for stronger international sanctions on North Korea, a day after Pyongyang said it had carried out a fourth nuclear test.

US President Barack Obama and South Korean President Park Geun Hye agreed to cooperate on a new UN resolution for strong sanctions, the president's office in Seoul said.

In a phone conversation on Thursday, Park and Japanese Prime Minister Shinzo Abe also vowed to work together to ensure the UN Security Council adopted sanctions, Yonhap News Agency reported.

The agreement between the two Asian countries - which have had a tense relationship but share a common enemy in North Korea - came in a 15-minute discussion between Abe and Park, Yonhap said.

In London, Britain's Foreign Office summoned the North Korean ambassador to "stress in the strongest terms the UK's condemnation of the nuclear test."

"This is a clear violation of multiple UN Security Council resolutions, and the United Kingdom supports the resolve of the UN to implement further significant measures against the regime," Asia Minister Hugo Swire after meeting the North Korean ambassador.

Britain is one of the five veto-holding permanent members of the Security Council, along with the United States, China, Russia and France.

"It is essential, as ever, that the international community is united in its approach and will work on such measures in a new Security Council resolution," Swire said.

He urged North Korea's leaders to "act in the best interests" of the country's 24.6 million people and "choose the path which will genuinely benefit them."

"Amid reports of widespread hardship and human rights abuses, the priority must be the health and welfare of North Korean people," Swire said.

On Wednesday, the UN Security Council in New York pledged to take "significant measures" against North Korea and said it would begin working on a resolution.

Each of North Korea's three previous atomic tests have resulted in a tightening of international sanctions.

Pyongyang said that Wednesday's test was a hydrogen bomb, a more powerful type of weapon than it had tested before.

Experts have however expressed doubts about whether the shock caused by the blast was powerful enough to have been from a hydrogen bomb.

North Korea's announcement provoked fury from its neighbours, as well as condemnation from around the world, including from China, Pyongyang's most important ally.

In response to the test, US Defence Secretary Ashton Carter reaffirmed his country's "ironclad commitment" to the security of South Korea.

"Any such test ... is both a flagrant violation of international law and a threat to the peace and stability of the Korean peninsula and the entire Asia-Pacific region," Carter and his South Korean counterpart Han Min Koo said in a statement.

The US guarantees South Korea's security under its nuclear umbrella, and has 28,500 soldiers stationed in the country.

Both sides wanted to coordinate "appropriate alliance responses," the Defence Ministry in Seoul said, without specifying details.

The head of US forces in South Korea, General Curtis Scaparrotti, and the chairman of the country's joint chiefs of staff, General Lee Sun Jin, had discussed the deployment of further US "strategic assets" to South Korea in the wake of the test, Yonhap reported Thursday.

Such assets could include a nuclear submarine or jet fighters.

In a related development, South Korea said Thursday that it would restart propaganda broadcasts via loudspeaker to the North. The broadcasts were stopped as part of a deal on defusing tensions signed with Pyongyang in August.

The broadcasts would begin on Friday at noon (0400 GMT), a presidential spokeswoman said.

Japan, which has long feared being a target of North Korean missiles, said that it should increase the sharing of military intelligence with South Korea in the wake of the test.

"It is extremely important that Japan and South Korea cooperate in the field of information, so as to better respond to North Korea's nuclear and missile programmes," deputy chief cabinet secretary Koichi Hagiuda told a news conference.

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South Korea urges China to join sanctions on North Korea

Worldwide anger over North Korean nuclear test

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