South Korea on Friday resumed high-decibel propaganda broadcasts into North Korea as the US ramped up pressure on China to bring Pyongyang to heel after its latest nuclear test.

While North Korean leader Kim Jong Un celebrated his 32nd birthday, the international community scrambled to find common ground on how best to penalise his regime following its shock announcement two days ago that it had successfully tested its first hydrogen bomb.

The cross-border broadcasts blare out an eclectic mix of everything from K-pop and weather forecasts to snippets of news and critiques of the North Korean regime.

Among the songs on Friday's playlist was Bang, Bang, Bang a recent hit by A-list K-pop boy band, Big Bang.

Their resumption revives psychological warfare tactics that date back to the 1950-53 Korean War. But they can be remarkably effective.

Their use during a dangerous flare-up in cross-border tensions last year infuriated Pyongyang, which at one point threatened artillery strikes against the loudspeaker units unless they were switched off.

The South finally pulled the plug after an agreement was reached in August to de-escalate a situation that had brought the two rivals to the brink of an armed conflict.

Now they are back - punishment for Wednesday's surprise nuclear test, which triggered global condemnation and concern, despite expert opinion that the yield was far too low to support the North's claim that the device was an H-bomb.

Diplomatic frenzy  

The test set off a diplomatic frenzy as the UN Security Council met to discuss possible sanctions and world leaders sought to build a consensus on an appropriate response to such a grave violation of UN resolutions.

Most eyes were on North Korea's main ally, China, which condemned the test but gave no signal that it was ready to approve a significant tightening of sanctions on its recalcitrant neighbour.

In a phone call with his Chinese counterpart Wang Yi on Thursday, US Secretary of State John Kerry insisted that Beijing's softly-softly line had failed and it was time to take a tougher stance with Pyongyang.

"Now China had a particular approach that it wanted to make and we agreed and gave them time to implement that," Kerry told reporters.

"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."

British Foreign Secretary Philip Hammond delivered a similar message during a visit to the USS Ronald Reagan aircraft carrier, docked at the Yokosuka Naval Base southwest of Tokyo.

"Continuing with words is not enough, we have to show we are prepared to take actions to ensure sanctions against North Korea are effective," Hammond said.

While Beijing has restrained US-led allies from stronger action against Pyongyang in the past, it has shown increasing frustration with its refusal to suspend testing.

But China's leverage over Pyongyang is mitigated, analysts say, by its overriding fear of a North Korean collapse and the prospect of a reunified, US-allied Korea directly on its border.

Allies confer  

On Thursday, President Barack Obama also spoke with the leaders of the two main US allies in Asia - and North Korean neighbours - South Korea and Japan.

The three countries, who have long sought to project a united front against the North Korean nuclear threat, agreed to work together at the UN to secure the strongest possible Security Council resolution.

North Korea, meanwhile, has said virtually nothing since its TV broadcast at noon on Wednesday announcing the "world startling event" of its latest test.

The test, personally ordered by leader Kim Jong Un, was of a miniaturised H-bomb, Pyongyang said, adding that it had now joined the ranks of "advanced nuclear nations".

The detonation came two days before Kim's birthday which passed Friday with no special mention in the state media, although the timing of the test was clearly aimed at burnishing his leadership credentials.

There is still widespread speculation over what device the North actually did test, but international experts mostly concur that it could not have been a full-scale thermonuclear device as claimed.

The yield from Wednesday's explosion was initially estimated at six kilotons, whereas a two-stage H-bomb would be expected to release 1,000 times more energy.

Nevertheless, the test was seen as a major cause for concern, with some experts suggesting it may have been a boosted-fission device, which would mark a significant step up from the North's three previous tests in 2006, 2009 and 2013.