SEOUL, South Korea (AP) — 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.

The announcement will likely draw a strong response from Pyongyang. North Korea sees the military drills as part of a U.S. plot to invade and becomes particularly upset about U.S. nuclear activities in the region. Washington and Seoul say the drills are routine and defensive.

North Korea has already threatened nuclear strikes on Washington and Seoul in recent weeks. It said Wednesday there was no need for communication in a situation "where a war may break out at any moment." Earlier this month, it announced that it considers void the armistice that ended the Korean War in 1953.

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But Pyongyang would have gone beyond words, possibly damaging its own weak finances, if it had blocked South Koreans from getting in and out of the Kaesong industrial plant, which produced $470 million worth of goods last year.

South Korean managers at the plant reported no signs of trouble Thursday.

Analysts see a full-blown North Korean attack as extremely unlikely, though there are fears of a more localized conflict, such as a naval skirmish in disputed Yellow Sea waters. Such naval clashes have happened three times since 1999.

The Kaesong plant, just across the heavily fortified Demilitarized Zone that separates the Koreas, normally relies on a military hotline for the governments to coordinate the movement of goods and South Korean workers.

Without the hotline, the governments, which lack diplomatic relations, used middlemen. North Korea verbally approved the crossing Thursday of hundreds of South Koreans by telling South Koreans at a management office at the Kaesong factory. Those South Koreans then called officials in South Korea.

Both governments prohibit direct contact with citizens on the other side, but Kaesong has separate telephone lines that allow South Korean managers there to communicate with people in South Korea.

Factory managers at Kaesong reached by The Associated Press by telephone at the factory said the overall mood there is normal.

"Tension rises almost every year when it's time for the U.S.-South Korean drills to take place, but as soon as those drills end, things quickly return to normal," Sung Hyun-sang said in Seoul, a day after returning from Kaesong. He is president of Mansun Corporation, an apparel manufacturer that employs 1,400 North Korean workers and regularly stations 12 South Koreans at Kaesong.

"I think and hope that this time won't be different," Sung said.

Technically, the divided Korean Peninsula remains in a state of war. North Korea last shut down communications at Kaesong four years ago, and that time some workers were temporarily stranded.

North Korea could be trying to stoke worries that the hotline shutdown could mean that a military provocation could come any time without notice.

South Korea urged the North to quickly restore the hotline, and the U.S. State Department said the shutdown was unconstructive.

North Korea's latest threats are seen as efforts to provoke the new government in Seoul, led by President Park Geun-hye, to change its policies toward Pyongyang. North Korea's moves at home to order troops into "combat readiness" also are seen as ways to build domestic unity as young leader Kim Jong Un, who took power after his father's death in December 2011, strengthens his military credentials.

The Kaesong complex is the last major symbol of inter-Korean cooperation. Other rapprochement projects created during a previous era of detente stopped as tension rose in recent years.

At the border Thursday, a trio of uniformed South Korean soldiers stood at one side of a gate as white trucks rumbled through, carrying large pipes and containers to Kaesong. At Dorasan station, a South Korean border checkpoint, a green signboard hung above the trucks with the words "Kaesong" and "Pyongyang" written in English and Korean.

The stalled hotline, which consists of two telephone lines, two fax lines and two lines that can be used for both telephone and fax, was virtually the last remaining direct link between the rival Koreas.

North Korea in recent weeks cut other phone and fax hotlines with South Korea's Red Cross and with the American-led U.N. Command at the border. Three other telephone hotlines used only to exchange information about air traffic were still operating normally Thursday, according to South Korea's Air Traffic Center.

In 2010, ties between the rivals reached one of their lowest points in decades after North Korea's artillery bombardment of a South Korean island and a South Korean warship sinking blamed on a North Korean torpedo attack. A total of 50 South Koreans died.

There is still danger of a confrontation or clash. Kim Jong Un may be more willing to take risks than his father, the late Kim Jong Il, said Yoo Ho-yeol, a North Korea expert at Korea University in South Korea.

Although North Korea has vowed nuclear strikes on the U.S., analysts outside the country have seen no proof that North Korean scientists have yet mastered the technology needed to build a nuclear warhead small enough to mount on a missile.

President Park so far has outlined a policy that looks to re-engage North Korea, stressing the need for greater trust while saying Pyongyang will "pay the price" for any provocation. Last week she approved a shipment of anti-tuberculosis medicine to the North.

Since 2004, the Kaesong factories have operated with South Korean money and know-how, with North Korean factory workers managed by South Koreans.

Inter-Korean trade, which includes a small amount of humanitarian aid sent to the North and components and raw materials sent to Kaesong complex to build finished products, amounted to nearly $2 billion in 2012, according to South Korea's Unification Ministry.

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Associated Press writer Youkyung Lee contributed to this report.