Ignoring threats of retaliation, the United Nations 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.

In an angry response, North Korea said Friday that it was nullifying all agreements of nonaggression and denuclearization with South Korea and was cutting off the North-South hot line. But beyond those steps, it was unclear how, if at all, North Korea’s young and untested leader, Kim Jong-un, would react 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 by the North.

Any military action, or response, could end up involving the American forces that have remained in South Korea as it has turned from war-ravaged ruin into one of the most advanced industrialized powerhouses.

The 15-to-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 includes 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. While China’s enforcement of sanctions on North Korea remains to be seen, it may now be more assertive.