But in an apparent attempt to ease those tensions, Defense Secretary Chuck Hagel said during a briefing that the B-2 bombers, which dropped dummy bombs on a South Korean island, were not intended to provoke North Korea.

“The United States and South Korea have not been involved in [provocations]. We, over the years, have been engaged with South Korea on joint exercises. The B-2 flight was part of that,” Mr. Hagel said, adding that the U.S. and other nations in the region are “committed to a pathway to peace.”

However, he added: “The North Koreans seem to be headed in a different direction here. So we will unequivocally defend, and we are unequivocally committed to that alliance with South Korea as well, as our other allies in that region of the world, and we will be prepared — we have to be prepared — to deal with any eventuality there.”

Mr. Hagel said the U.S. takes seriously the threats posed by North Korea’s secretive, totalitarian regime and that there are “a lot of unknowns” about its young leader, Kim Jong-un.

The Pentagon announced this month that it is increasing its missile interceptors in Alaska, deploying a second early warning system in Japan and studying whether to add ground-based missile sites in Alaska to deal with the North Korean threat.

Also, an additional U.S. aircraft carrier is in the region. The USS John C. Stennis recently arrived on its way home from the Middle East, joining the USS George Washington, which is permanently deployed to Japan.

The U.S. also has 28,500 troops stationed in South Korea.

Military analysts said including B-2 stealth bombers in the joint military exercises was a show of power that sent a clear message about U.S. resolve on the peninsula.

“It was a highly publicized move that was clearly intended as a sign to North Korea as a deterrent to North Korean provocation,” said Bruce Klingner, senior research fellow on Northeast Asia at the Heritage Foundation. “The move also comes at a time when some in South Korea are questioning U.S. military capabilities and resolve given massive budget cuts to the Pentagon budget.”

Although the Pentagon downplayed the move, Mr. Klingner said U.S. concerns over an outbreak of war between the two Koreas are very real.

On Wednesday, North Korea cut a third telephone hotline — a military line — to the South, after announcing Tuesday that it was placing its missile and artillery batteries at “full combat readiness,” meaning the units are “set to strike the bases of the U.S. imperialist forces of aggression on the U.S. mainland and in the Pacific … including Hawaii and Guam.”

North Korea said this month that it was scrapping the cease-fire pact that ended fighting in the Korean War in 1953, and it threatened a “pre-emptive” nuclear strike on the United States.

The provocations have come in response to the U.N. imposing new sanctions on North Korea to punish the rogue state for an illegal nuclear test in February and to ongoing U.S.-South Korean military exercises in the region.

Mr. Klingner said Mr. Kim has been emboldened by the nuclear test — the North’s largest to date — and its successful launch of a three-stage rocket in December that Western observers said was a cover for a ballistics missile test.

He noted that neither Seoul nor Washington struck back at Pyongyang in 2011 after the North shelled an island controlled by the South, killing four people, and after a South Korean warship exploded and killed 46 sailors. An international probe determined that a North Korean torpedo sank the ship, though Pyongyang denied any involvement.

South Korea’s new president, Park Geun-hye, has vowed to respond in kind to any North Korean aggression and reiterated her campaign vow of reaching out to the impoverished communist state.

“Some in South Korea have called for Seoul to develop nuclear weapons or to call for Washington to reintroduce tactical nuclear weapons that had been removed from the peninsula in the 1990s,” Mr. Klingner said. “The fear always has been that even the restrained tactical response would lead to full-blown war.”

Analysts have dismissed North Korea’s threats against the U.S. mainland as empty rhetoric, saying the regime’s long-range missiles cannot reach the West Coast.

“North Korea doesn’t have the capability to carry out this latest threat to attack U.S. bases in Hawaii, the U.S. mainland or Guam using long-range missiles,” said James Hardy, Asia-Pacific editor of IHS Jane’s Defense Weekly.

Analysts generally say a full-blown North Korean attack on the South is 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.