UNITED NATIONS — Ignoring threats of retaliation, the UN Security Council ordered new economic sanctions against North Korea on Thursday for its third nuclear test last month, unanimously approving a resolution that the United States negotiated with China, the North’s greatest protector.
It was unclear how, if at all, North Korea’s young and untested leader, Kim Jong Un, would respond to the rebuke. His government has threatened to terminate the 60-year-old armistice that brought a halt to the Korean War and that has kept a cold peace on the peninsula since, and South Korean officials said they were on the alert for any possible attack as the North seeks to vent its anger.
Any such action, or response, could end up involving US forces that have remained in South Korea as it has turned from war-ravaged ruin into one of the world’s most advanced industrialized powerhouses.
The 15-0 Security Council vote places potentially painful new constraints on North Korean banking, trade and travel, pressures countries to search suspect North Korean cargo and contains new enforcement language absent from previous measures.
But the provisions are in some ways less important than China’s participation in writing them, suggesting that the country has lost patience with the neighbor it supported in the Korean War, and that it may be more assertive going forward in seeking to pressure the North Koreans.
‘’This is not about the words, it is about the music,’’ said Christopher R. Hill, the former US diplomat who negotiated a deal with the North during the George W. Bush administration to dismantle its nuclear facilities — an accord that quickly collapsed.
China’s co-sponsorship of the resolution ‘‘suggests that after many years, the screws are beginning to turn,’’ said Hill, now the dean of the Josef Korbel School of International Studies at the University of Denver.
The UN vote came hours after North Korea, infuriated by the combination of the proposed resolution and continuing annual joint military exercises by South Korea and the United States, threatened for the first time to carry out ‘‘a preemptive nuclear strike’’ on its enemies, of which the United States ranks first.
Military specialists regarded that threat as bluster: While the North has conducted three underground nuclear tests, it is far from clear it knows how to deploy a nuclear weapon or make one small enough to fit atop a missile.
But the threat still prompted the White House spokesman, Jay Carney, to respond that the United States was ‘‘fully capable’’ of defending itself.
Another nuclear test is possible, as is another ballistic missile launching or perhaps an armed provocation aimed at South Korea, where a new president, Park Geun-hye, the daughter of a former South Korean dictator who was known for taking a hard stand with the North, could be forced to respond.
Some regarded the North’s dire warnings as a signal that some military response was looming.
On the other hand, Richardson said, ‘‘China is part of a significant sanctions effort, and this may cool the North Koreans down, may temper their response.’’
The resolution passed Thursday instructs North Korea to cease all nuclear and missile testing and contains new restrictions that will block financial transactions, impound cash, further empower other countries to inspect suspicious North Korean cargo and expand a blacklist of items that the country is prohibited from importing.
But the restrictions do not allow countries to inspect North Korean shipments on the high seas. The sanctions also place constraints on North Korean diplomats, raising their risk of expulsion.