North Korea on Wednesday threatened to launch military strikes against South Korea if any of its ships were stopped or searched as part of an American-led operation to intercept vessels suspected of carrying weapons of mass destruction.

South Korea agreed to join the global interdiction programme after North Korea tested a nuclear device on Monday. The North had earlier warned the South not to participate in the effort, known as the Proliferation Security Initiative.

“We consider this a declaration of war against us,” an unidentified North Korean military spokesman said on Wednesday in a statement carried by the North’s 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 strident rhetoric, although not unusual in North Korean statements released to the outside world, is likely to further sharpen tensions created by the North’s surprise nuclear test, which drew a condemnation that was swift, widespread and angry.

Earlier on Wednesday, a South Korean newspaper reported that American spy satellites had detected plumes of steam and other signs of activity at a North Korean plant that reprocesses spent nuclear fuel to make weapons-grade plutonium. The report from the newspaper, Chosun Ilbo, appeared to support a claim made by North Korea in late April that it had restarted its reprocessing plant at Yongbyon, 60 miles north of Pyongyang, the capital.

In its statement on Wednesday, the North Korean military also questioned the “legal status” of five South Korean-held islands on the countries’ disputed western sea border. The military “will not guarantee the safe navigation” for US and South Korean vessels, military or civilian, sailing in the waters near the border, the spokesman said.