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

In Seoul, the government also 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 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.

Their consultations followed a meeting of the 15-member United Nations Security Council in New York City, which, with backing from China, Pyongyang's sole major ally, strongly condemned the test, and said it would begin work on a new U.N. draft resolution that would contain "further significant measures."

U.N. diplomats confirmed that talks were under way on strengthening several sets of sanctions that have been imposed on secretive North Korea since it first tested an atomic device in 2006. 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 U.S. 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 U.N. Security Council," it added. Later, the Blue House announced it would resume propaganda broadcasts using batteries of giant speakers along the border with North Korea from noon (0300 GMT) on Friday.

Japanese Prime Minister Shinzo Abe also spoke with Obama yesterday, and agreed that 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. Park and Abe also spoke by phone and made similar pledges to work together inside the U.N. Security Council.

In announcing that it had successfully tested a hydrogen bomb, North Korea said it had "joined the rank of advanced nuclear states" like Russia, France and the U.S. 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."

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. But experts said the explosive yield from Wednesday's test – initially estimated at between 6 and 9 kilotons – was far too small. Japan said three planes it sent up Wednesday to try and collect traces of radioactive material that might help clarify the nature of the test, had returned empty-handed.

At the U.N., U.S. Ambassador Samantha Power called for a "tough, comprehensive and credible package of new sanctions" to make clear to Pyongyang that there are "real consequences" to its actions.

All eyes at the U.N. 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 neighbor. But Beijing's leverage over Pyongyang is mitigated, analysts say, by its overriding fear of a North Korean collapse and the prospect of a reunified, U.S.-allied Korea directly on its border.

There was no immediate response from North Korea to the U.N. sanctions threat, but its KCNA official news agency was unrepentant. "The more frantic the hostile forces get in their moves to isolate and stifle the DPRK [North Korea], the stronger its nuclear deterrent will grow," it said in a commentary.