UNITED NATIONS — A united U.N. Security Council strongly condemned North Korea’s nuclear test and pledged further action Tuesday, calling Pyongyang’s latest defiant act “a clear threat to international peace and security.”

All 15 council members, including North Korea’s closest ally China, approved the press statement hours afterthe latest underground test. The statement called the atomic blast a “grave violation” of three U.N. resolutions that ban North Korea from conducting nuclear or missile tests.

The swift and unanimous response from the U.N.’s most powerful body set the stage for a fourth round of sanctions against Pyongyang.

The language in the statement, which pledged “significant action” in a new resolution, was stronger than the Security Council’s initial reactions to North Korea’s two previous nuclear tests and to its most recent rocket launch. Diplomats said it reflects the growing anger at Pyongyang’s continuing defiance of the council and the sanctions resolutions.

How tough the new sanctions will be will depend largely on China, North Korea’s main trading partner. China has voted for the three previous sanctions resolutions but has resisted measures that would cut off the country’s economy completely.

U.S. Ambassador to the U.N. Susan Rice said a number of new measures to tighten and expand the existing sanctions will be discussed with council members and other concerned countries in the coming days. She added in response to a question that financial sanctions would be “right for appropriate further action.”

Tuesday’s nuclear test followed a familiar North Korean pattern. Pyongyang conducted its first two nuclear weapons tests weeks after rocket launches in 2006 and 2009, and this third test followed a rocket launch in December that the U.N. and the U.S. called a cover for a banned long-range missile test.

The Security Council noted that last month, in a resolution that strengthened sanctions in response to the December rocket test, its members promised to take “significant action” in the event of a new nuclear test.

“In line with this commitment and the gravity of this violation, the members of the Security Council will begin work immediately on appropriate measures in a Security Council resolution,” the council said.

The statement was read by South Korea Foreign Minister Kim Sung-hwan, whose country holds the rotating Security Council presidency this month. South Korea was elected to the council for a two-year term starting in January.

“North Korea will be held responsible for any consequences of this provocative act,” Kim told reporters later.

Rice called the test “highly provocative” and said the North’s continued work on its nuclear and missile programs threatens regional and international peace and “the security of a number of countries including the United States.”

“They will not be tolerated,” she said, “and they will be met with North Korea’s increasing isolation and pressure under United Nations sanctions.”

“The U.N. Security Council must and will deliver a swift, credible and strong response by way of a Security Council resolution that further impedes the growth of DPRK’s nuclear-weapons and ballistic-missile programs and its ability to engage in proliferation activity,” Rice said, using the initials of the North’sofficial name, the Democratic People’s Republic of Korea.

U.N. Secretary-General Ban Ki-moon welcomed the council statement and said he was encouraged by “the swift and overwhelming international condemnation of this wanton act.”

Ban, a former South Korean foreign minister, expressed regret that his repeated appeals to North Korea’s new young leader, Kim Jong Un, to give up its pursuit of nuclear weapons and address international concerns through dialogue “have fallen on deaf ears.”

North Korea is the only country to carry out nuclear tests in the 21st century, he said, and the latest test in defiance of the U.N. and the international community “is a serious challenge to global efforts to curb nuclear proliferation.”

Ban said he is also “profoundly concerned about the negative impact of this act on regional stability.” OBAMA’S REACTION

President Barack Obama reacted sharply to NorthKorea’s nuclear test Tuesday, promising swift international action to bring the rogue communist regime in line.

In a statement, Obama called Pyongyang’s test a “highly provocative act” that threatens U.S. security and international peace. The reaction from the White House was significantly stronger than it was after North Korea’s rocket test in December, when the administration only promised “appropriate action” alongside America’s allies.

“The danger posed by North Korea’s threatening activities warrants further swift and credible action by the international community,” Obama said in a statement early Tuesday. “The United States will also continue to take steps necessaryto defend ourselves and our allies.”

Secretary of State John Kerry spoke by telephone with the foreign ministers of South Korea and Japan after the test, as well as China, and Obama called South Korean President Lee Myung-bak to assure him the U.S. would help defend his country.

North Korea’s official Korean Central News Agency said the test was conducted safely but with “great explosive power.” The test counters the “ferocious” U.S. hostility that undermines the North’s peaceful, sovereign right to launch satellites, it argued. Last month, North Korea’s National Defense Commission said the United States was its prime target for a nuclear test and longrange rocket launches.

“These provocations do not make North Korea more secure,” Obama said. “Far from achieving its stated goal of becoming a strong and prosperous nation, North Korea has instead increasingly isolated and impoverished its people through its ill-advised pursuit of weapons of mass destruction and their means of delivery.”

The U.S. already maintains severe unilateral sanctions against North Korea, and commerce between the two countries is nearly nonexistent. Tougher global sanctions are dependent on the participation of China, Pyongyang’s primary trading partner.

More forceful U.S. consequences, in the form of a military response, are highly unlikely even thoughthe United States remains technically at war with the North Koreans. Only the 1953 armistice ending the Korean War keeps the U.S. and the North from hostilities, and some 28,500 U.S. troops in South Korea to deter potential aggression.

CHINA’S STRONG WORDS

Chinese Foreign Minister Yang Jiechi summoned North Korea’s ambassador to China on Tuesday to tell him Beijing was “strongly dissatisfied and resolutely opposed” to North Korea’s nuclear test, the Foreign Ministry said.

China urged its longtime ally “to honor its commitment to denuclearization and not to take action that may worsen the situation.”

Chinese leaders have spent recent weeks trying strenuously to dissuade Pyongyang from the nuclear test, according to Western diplomats speaking on condition of anonymity. Their failure to do so points to growing exasperation in Beijing with North Korean leader Kim and reflects a deteriorated relationship between the two countries that could have global consequences, the diplomats said.

For years, diplomats and experts have thought that if anyone could persuade North Korea, it was the Chinese. Long seen as a key factor in propping up the regime in Pyongyang, China has come under considerable international pressure to use its influence to push North Korea toward denuclearization and away from provocations.

All the while, China has maintained fairly stalwart support for North Korea - by watering down international sanctions and sending desperately needed aid - even as its leaders tried to convince countries such as the United States that they have limited influence. That assertion seems to have been borne out by Tuesday’s nuclear test.

“The nuclear test of North Korea this time will push China to rethink,” said Zhu Feng, a North Korea expert at Peking University in China. “In the past, China always thought that the nuclear issue of North Korea could be solved by communication and negotiation. But this time, North Korea showed their determination and hard line to own nuclear weapons, which is not negotiable.” Information for this article was contributed by Edith M. Lederer, Bradley Klapper, Lolita C. Baldor and Kimberly Dozier of The Associated Press and by William Wan and Zhang Jie of The Washington Post.

Front Section, Pages 1 on 02/13/2013