SEOUL, South Korea — In a show of force following weeks of North Korean bluster, the U.S. on Thursday took the unprecedented step of announcing that two of its nuclear-capable B-2 bombers joined joint military drills with South Korea, dropping dummy munitions on an island range.

The announcement is likely to further enrage Pyongyang, which has already issued a flood of ominous statements to highlight displeasure over the drills and U.N. sanctions over its nuclear test last month. But there were signs Thursday that it is willing to go only so far.

A North Korean industrial plant operated with South Korean know-how was running normally, despite the North’s shutdown a day earlier of communication lines ordinarily used to move workers and goods across the border. At least for the moment, Pyongyang was choosing the factory’s infusion of hard currency over yet another provocation.

U.S. Forces Korea said in a statement that the B-2 stealth bombers flew from a U.S. air base in Missouri and dropped dummy munitions on the South Korean island range before returning home.

It was unclear whether America’s stealth bombers were used in past annual drills with South Korea, but this is the first time the military has announced their use.

The statement follows an earlier U.S. announcement that nuclear-capable B-52 bombers participated in the joint military drills.

In Washington, State Department spokeswoman Victoria Nuland said the U.S. was making sure its defenses were “appropriate and strong” as North Korea continues to test and seeks to extend the reach of its weaponry.

“When a country says the kind of things that the DPRK is saying, you have to take it seriously,” Nuland told reporters Thursday, referring to the North by its formal name, the Democratic People’s Republic of Korea.