The UN Security Council on Wednesday agreed to roll out new measures to punish North Korea after Pyongyang said it carried out a successful hydrogen bomb test -- a claim rejected by Washington and experts.

With backing from China, Pyongyang's sole major ally, the 15-member council 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 secretive North Korea since it first tested an atomic device in 2006.

Pyongyang's announcement drew swift condemnation from the international community, including from China and Washington, which said an initial analysis was "not consistent" with North Korea's claims of a successful hydrogen bomb test.

South Korean President Park Geun-Hye condemned what she described as a "grave provocation" and called for a strong international response.

"I demand the DPRK cease any further nuclear activities," UN Secretary General Ban Ki-moon said.

Skeptical experts suggested the apparent yield was far too low for a thermonuclear device.

North Korean state television announced "the republic's first hydrogen bomb test," which was "successfully performed at 10:00 am (0130 GMT)."

"We have now joined the rank of advanced nuclear states," it said, adding that the test was of a miniaturized device.

State television showed North Korean leader Kim Jong-Un's signed order -- dated December 15 -- to go ahead with the test and begin 2016 with the "thrilling sound of the first hydrogen bomb explosion."

- 'Clear threat to peace' -

After meeting behind closed doors, the UN Security Council "strongly condemned" the underground test and described it as a "clear threat to international peace and security."

US 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.

Japanese Ambassador Motohide Yoshikawa said he will be pushing for "a series of measures under chapter 7" of the UN charter, which provides for enforceable sanctions.

But it remained an open question whether China, a veto-wielding council member, would support tough measures against its ally.

The foreign ministry in Beijing said it "firmly opposes" the nuclear test, which was carried out "irrespective of the international community's opposition."

The three previous tests in 2006, 2009 and 2013 triggered waves of UN sanctions.

Currently, there are a total of 20 entities and 12 individuals on the UN sanctions blacklist, which provides for a global travel ban and an assets freeze.

In 2013, the council ramped up the sanctions by asking all countries to prevent the sale of luxury goods to North Korea, a measure designed to hit at Pyongyang's elites.

- Skepticism -

The new test, which came just two days before Kim's birthday, was initially detected as a 5.1-magnitude tremor at the North's main Punggye-ri nuclear test site in the northeast of the country.

The explosive yield was initially estimated at between six and nine kilotons -- similar to the North's last nuclear test in 2013.

Most experts had assumed North Korea was years from developing a hydrogen bomb, and assessments were divided on how far it had gone in developing a miniaturized warhead to fit on a ballistic missile.

"The initial analysis that has been conducted... is not consistent with North Korea's claim of a successful hydrogen bomb test," White House spokesman Josh Earnest said.

"There is nothing that has occurred in the last 24 hours that has caused the United States government to change our assessment of North Korea's technical and military capabilities."

Bruce Bennett, a senior defense analyst with the Rand Corporation research group, said if an H-bomb was actually tested, the detonation clearly failed -- at least the fusion stage.

"If it were a real H-bomb, the Richter scale reading should have been about a hundred times more powerful," Bennett told AFP.

- China key -

North Korea's fourth nuclear test marked a striking act of defiance in the face of warnings from enemies and allies that Pyongyang would pay a steep price for moving forward with its nuclear weapons program.

The final response of China, North Korea's economic and diplomatic patron, will be key in determining the international community's next step, experts say.

"Beijing will face increased pressure both domestically and internationally to punish and rein in Kim Jong-Un," said Yanmei Xie, the International Crisis Group's senior analyst for Northeast Asia.

"A nuclear-armed North Korea is uncomfortable and disturbing.

"But a regime collapse in Pyongyang leading to mass chaos next door and potentially a united Korean peninsula with Washington extending its influence northward to China's doorstep is downright frightening."

Roberta Cohen of the US Brookings think tank called China "the elephant in the room and what it is willing to do unilaterally as well as in the UN."

"China really has to decide whether North Korea as it stands now is a liability or an asset," she said.