(CNN) North Korea claimed it had successfully conducted its fifth and biggest nuclear test on Friday -- and reaction from around the world has been almost as explosive.

World leaders lined up to condemn the blast Friday which was estimated to have an explosive power of 10 kilotons, almost twice as large as the previous test, according to Kim Nam-wook of South Korea's Meteorological Administration.

But what happens next? And who is in a position to lay down the law to North Korea and its nuclear program? Its closest ally China is reluctant to go further than a statement of condemnation and Pyongyang is already subject to the strictest sanctions in the world. There are few options left, short of cutting off the power supplies -- in effect, turning off the lights in North Korea.

What are the neighbors saying?

North Korea has tested five nuclear missiles over the past 10 years -- and the rest of the world appears to be helpless to stop it.

This was the biggest and most powerful yet -- leading to South Korean President Park Geun-hye labeling Kim Jong Un's regime "fanatically reckless."

"The only thing that Kim Jong Un regime can gain from the nuclear tests is stronger sanctions from the international community and its isolation," she said in a statement.

"Such provocation will eventually hasten its path to self-destruction."

She later added: "The state of mind of Kim Jong Un, who is not listening to the international society and the surround countries because he wants to maintain his power, should be viewed as uncontrollable."

China caution

China is seen as North Korea's only major ally and all eyes are now on Beijing to see what happens next. But this latest incident could add pressure to the relationship.

Chinese foreign ministry spokeswoman Hua Chunying declined to say if China had received advanced notice of North Korea's nuclear warhead test Friday.

She said China would summon DPRK embassy representatives to make Beijing's views known, but declined to say if it would support new tougher sanctions.

She added that "all sides' security concerns should be addressed and any unilateral actions would only lead a vicious cycle."

China joined the rest of the international community in approving tough new sanctions on North Korea in March.

Those sanctions prevented Chinese banks from operating in North Korea and cracking down on the export of military or dual use materials.

Japanese protest

The latest test claimed by North Korea comes just days Pyongyang test-fired three ballistic missiles, which landed in Japan's Air Defense Identification Zone, about 155 miles (250 kilometers) from a Japanese island.

Last month, a missile fired from a submarine also landed in the same zone.

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Japanese Prime Minister Abe told reporters Friday that if North Korea had conducted another nuclear test it was "absolutely unacceptable."

"We must lodge a strong protest," he said.

Japanese Foreign Minister Fumio Kishida told reporters in Tokyo that Japan is studying further unilateral sanctions against North Korea, but did not elaborate on exactly what those measures would be.

US reaction

US President Barack Obama was briefed by national security adviser Susan Rice on the situation and spoke to both Park and Japanese Prime Minister Shinzo Abe, White House press secretary Josh Earnest told pool reporters aboard Air Force One on the US leader's return trip from Asia.

Earnest said the President spoke separately to South Korean and Japanese leaders by phone, reassuring them of "the unbreakable US commitment to the security of our allies in Asia and around the world."

Later Friday, Obama released a statement, which labeled the nuclear test as "a grave threat to regional security and to international peace and stability."

He added: "To be clear, the United States does not, and never will, accept North Korea as a nuclear state."

Russia "concerned"

Russia's foreign minister Sergey Lavrov denounced the actions of North Korea and urged for the resolutions of the UN Security Council to be "strictly implemented."

"This defiance of the international law norms and the world community's opinion deserves the strongest condemnation," a statement from the Russian foreign ministry read.

"North Korea's actions, aimed at undermining the global nonproliferation regime, pose a serious threat to peace and security on the Korean peninsula and in the Asia-Pacific region.

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"This will have negative consequences primarily for North Korea itself.

"We insist that the North Korean side should stop its dangerous adventures, strictly comply with all the UN Security Council requirements, completely abandon its nuclear missile programs and return to the Treaty on the Non-Proliferation of Nuclear Weapons."

What happens next?

The United Nations will discuss its next move at a meeting Friday.

North Korea is the most heavily sanctioned country in the world but such conditions have yet to dissuade Pyongyang from abandoning its nuclear campaign.

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North Korea was hit with a whole list of sanctions in March which included the prohibition of supplying aviation fuel, including rocket fuel, and the sale of small arms, to Pyongyang.

The sanctions included a ban on North Korea exporting most of the country's natural resources with coal alone estimated to be worth $1 billion in annual income, according to Samantha Power, the US ambassador to the UN.

In July, the Obama administration hit Kim Jong Un and 10 other regime officials with sanctions for their alleged complicity in human rights abuses against the North Korean people.

The move marked the first time Washington sanctioned the nation's leader personally.

China could be pressed to take the strongest possible action by blocking the transportation of fuel and oil but that could have grave consequences for the general population.

Such a move would be hugely controversial -- but options are running out.