Seoul (AFP) - The United States and its two main military allies in Asia, South Korea and Japan, pledged a combined push Thursday to secure a comprehensive, hard-hitting international response to North Korea's latest nuclear test.

The leaders of the three countries, who have long sought to project a united front against the North Korean nuclear threat, spoke by phone a day after Pyongyang's shock announcement that it had tested its first hydrogen bomb.

While the announcement prompted widespread condemnation and calls for new stiff sanctions against the secretive state, it was also greeted with some scepticism, with experts suggesting the apparent yield was far too low for a thermonuclear device.

In Seoul, the government took unilateral action by announcing the resumption of high-decibel propaganda broadcasts into the North -- a tactic that had prompted Pyongyang to threaten military strikes when it was last employed during a cross-border crisis last year.

The consultations between the US, Japan and South Korea followed a meeting of the 15-member UN Security Council in New York which, with backing from China, Pyongyang's sole major ally, strongly condemned the test and said it would begin work on a new UN draft resolution that would contain "further significant measures".

UN diplomats confirmed that talks were under way on strengthening several sets of sanctions that have been imposed on North Korea since it first tested an atomic device in 2006.

- Anger in South Korea -

In South Korea, the mood was uncompromising, with President Park Geun-Hye calling for a strong international response to what she called a "grave provocation".

Park spoke with US President Barack Obama on Thursday morning, with both leaders insisting that the test merited the "most powerful and comprehensive sanctions," her presidential office said in a statement.

"The two leaders agreed that the North should pay the appropriate price... and vowed to closely cooperate to get a strong resolution adopted at the UN Security Council," it added.

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The White House, for its part, condemned North Korea's "latest reckless behaviour".

"President Obama reaffirmed the unshakeable US commitment to the security of the ROK (Republic of Korea)," the statement said, using the acronym for South Korea's official name.

Seoul said it would resume propaganda broadcasts using batteries of giant speakers along the border with North Korea from noon (0300 GMT) on Friday.

The move is likely to infuriate Pyongyang which, during an extended and increasingly hostile cross-border stand-off last year, had issued Seoul with an ultimatum to halt the broadcasts or face imminent attack.

The South only unplugged the speakers following a compromise accord reached on August 25.

Japanese Prime Minister Shinzo Abe also spoke with Obama on Thursday and agreed they should spearhead the effort to impose harsher penalties on Pyongyang.

"We will take firm and resolute steps, including considering measures unique to our nation," Abe said, hinting at unilateral moves.

Meanwhile, Britain summoned the North Korean ambassador to stress its condemnation of the nuclear test.

- All eyes on China -

Park and Abe also spoke by phone and made similar pledges to work together under the aegis of the UN Security Council.

The censure and sanctions threats had a familiar ring, given similar outrage that greeted the North's previous tests in 2006, 2009 and 2013, and some voices stressed the need to find a strategy that combined coercion with negotiation.

"A priority must be to find ways to both further pressure North Korea to limit its nuclear weapons capabilities and engage it diplomatically," said David Albright, president of the Washington-based Institute for Science and International Security.

All eyes at the UN will now be on China, a veto-wielding council member, to see just how far it will go in tightening the sanctions grip on its recalcitrant neighbour.

US Secretary of State John Kerry Thursday urged China to take a tougher line, saying Beijing's cautious approach had not borne fruit.

"Now China had a particular approach that it wanted to make and we agreed and give them time to implement that," he said, describing his call to Foreign Minister Wang Yi.

"But today in my conversation with the Chinese I made it very clear that that has not worked and we cannot continue business as usual."

Acquisition of a working H-bomb -- with a destructive power that dwarfs the bombs it has tested in the past -- would represent a massive leap forward in the North's nuclear weapons capability.

In announcing that it had successfully tested a hydrogen bomb, North Korea said it had "joined the rank of advanced nuclear states" such as Russia, France and the US that also boast thermonuclear devices.

The order for the test was personally signed by leader Kim Jong-Un, with a handwritten message to begin 2016 with the "thrilling sound of the first hydrogen bomb explosion".