South Korea agreed to join the operation after North Korea tested a nuclear device on Monday, its second nuclear test in three years. The North had earlier warned the South not to participate in the operation, known as the Proliferation Security Initiative.

“We consider this a declaration of war against us,” North Korea said in a statement carried by its official news agency, KCNA. “Any hostile act against our peaceful vessels, including search and seizure, will be considered an unpardonable infringement on our sovereignty, and we will immediately respond with a powerful military strike.”

The North Koreans also said in the statement that they “no longer feel bound by the armistice” that ended the fighting in the 1950-53 Korean War. Technically, the two Koreas have remained at war for more than 50 years, because the 1953 armistice was never replaced with a final peace treaty. The North Koreans had previously called the armistice a “useless piece of paper” and declared that they no longer felt bound by it. But they have rarely used the threat of abandoning the armistice.

At the United Nations, the United States and Japan were drawing up a rough draft of a Security Council resolution that would concentrate on five or six ways to flesh out existing sanctions against North Korea that had never been enforced, diplomats said. Although China supports the idea of sanctions, it wants to work slowly and to bolster measures first passed in 2006 rather than creating new ones, they said.

The proposals include banning imports and exports of all arms  only heavy weapons are restricted now. “We want to dry out their resources for the military,” said a senior Western diplomat, speaking anonymously because of the sensitivity of the negotiations.