* Western powers, South Korea want new sanctions on North Korea
* China expected to resist tough new sanctions -diplomats
* China, U.S., Russia all condemn North Korea's new test (Adds details, diplomat)
By Louis Charbonneau
UNITED NATIONS, Feb 12 (Reuters) - The U.N. Security Council strongly condemned North Korea's third nuclear test on Tuesday and vowed to take action against Pyongyang for an act that all major world powers, including traditional ally China, have denounced.
"The members of the Security Council strongly condemned this test, which is a grave violation of Security Council resolutions," South Korean Foreign Minister Kim Sung-hwan, whose country is president of the council this month, told reporters. He said the council would now consider "appropriate measures."
The non-binding statement was approved by all 15 council members.
U.S. Ambassador to the United Nations Susan Rice said that Washington and its allies want the Security Council to pass a resolution that would "augment the sanctions regime" already in place due to Pyongyang's 2006 and 2009 atomic tests.
The council statement was agreed at an emergency closed-door session convened by South Korea. Diplomats say negotiations on new sanctions could take weeks since China is likely to resist tough new measures for fear that new sanctions could lead to further retaliation by the North Korean leadership.
Beijing, has supported all previous sanctions resolutions against Pyongyang but only after working hard to dilute proposed measures in negotiations on the texts. It has been concerned that tougher sanctions could further weaken the North's economy and prompt refugees to flood into China.
The United States, Britain, France, Russia and China - the five permanent members of the 15-nation council - all condemned Pyongyang's latest nuclear test, which an international nuclear test monitoring agency in Vienna said was roughly twice as large as that of North Korea's 2009 nuclear test.
U.S. President Barack Obama said "the danger posed by North Korea's threatening activities warrants further swift and credible action by the international community."
Obama will give his annual State of the Union address later on Tuesday. Several diplomats said it was possible that was why North Korea chose this day to detonate an atomic device, since Pyongyang traditionally makes such moves on important days in the U.S. calendar.
China's reaction was more muted. In Beijing, Chinese Foreign Minister Yang Jiechi on Tuesday summoned the North Korean ambassador to China to protest against the North's new nuclear test. Yang said China was "strongly dissatisfied and resolutely opposed" to the test.
Several council diplomats said the United States and its allies would push for tough new sanctions rather than merely expanding measures imposed after Pyongyang's 2006 and 2009 atomic tests.
AFFRONT TO CHINA?
It was not clear what measures China would support. Diplomats said the Chinese delegation indicated behind closed doors that it could support a new council resolution on North Korea but did not specify what it would allow to be in it.
"Of course we'll go for sanctions," a council diplomat told Reuters. "As far as what kind of sanctions China could accept, this will be a Chinese-U.S. bilateral issue. Whatever they agree is what we'll be presented with in a resolution."
When North Korean leader Kim Jong-un, 30, took power after the death of his father Kim Jong-il in December 2011, there were hopes he would bring reforms and end "military first" policies.
Instead, the North Korea, whose economy is smaller than it was 20 years ago and where a third of children are believed to be malnourished, appears to be trapped in a cycle of sanctions followed by further provocations.
China has long made clear its opposition to a new North Korean nuclear test and U.N. diplomats predicted that it would most likely approve new sanctions eventually, although it is not clear how harsh Beijing will permit the measures to be.
Victor Cha of the Center for Strategic and International Security in Washington said "Chinese leaders cannot be happy with this test."
"In the world of diplomacy, little things do matter and conducting the test during the Chinese New Year will be viewed by Beijing as extremely insulting, and perhaps will lead them to take quiet punitive but temporary measures," wrote Cha, a former North Korea negotiator under President George W. Bush.
Chinese New Year began on Sunday.
"Whatever North Korea says, the nuclear test will gravely damage China's strategic and security interests," said Wang Dong, a professor at Peking University. "But the problem is that China's options, like the United States and others, are limited. North Korea is not our puppet. We cannot control its behavior."
In January the Security Council passed a resolution expanding U.N. sanctions against North Korea due to its December rocket launch and warned Pyongyang against further launches or nuclear tests. North Korea responded by threatening a new atomic detonation.
North Korea's previous nuclear tests prompted the Security Council to impose sanctions that include a ban on the import of nuclear and missile technology, an arms embargo and a ban on luxury goods imports.
There are 17 North Korean entities, including banks and trading companies, on the U.N. blacklist, and nine individuals - all linked to North Korea's nuclear and missile programs. U.N. diplomats say many more entities and individuals could be subject to international asset freezes and travel bans.
In a statement, U.N. Secretary-General Ban Ki-moon, a former South Korean foreign minister, said hoped the Security Council would take appropriate action.
"It is deplorable that Pyongyang defied the strong and unequivocal call from the international community to refrain from any further provocative measures," he said.
Ban later told the council authorities in Pyongyang "should not be under any illusion that nuclear weapons will enhance their security. To the contrary, as Pyongyang pursues nuclear weapons it will suffer only greater insecurity can isolation." (Additional reporting by Michael Martina and Ben Blanchard in Beijing and Michelle Nichols and Paul Eckert in Washington; Editing by Bill Trott and David Brunnstrom)