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Now this new sense of vulnerability is causing some influential South Koreans to break a decades-old taboo by openly calling for the South to develop its own nuclear arsenal, a move that would raise the stakes in what is already one of the world's most militarized regions.

While few here think this will happen anytime soon, two recent opinion polls show that two-thirds of South Koreans support the idea posed by a small but growing number of politicians and columnists — a reflection, analysts say, of hardening attitudes since the Feb. 12 underground nuclear test, the North's third such test since 2006.

"The third nuclear test was for South Korea what the Cuban missile crisis was for the U.S.," said Han Yong-sup, a professor of security policy at the Korea National Defense University in Seoul. "It has made the North Korean threat seem very close and very real."

In recent weeks, the North has approached a crucial threshold with its weapons programs, with the successful launching of a long-range rocket, followed by the test detonation of a nuclear device that could be small enough to fit on top of a rocket. Those advances have been followed by a barrage of apocalyptic threats to rain "pre-emptive nuclear strikes" and "final destruction" on Seoul, the South's neon-drenched capital. This intensification of North Korea's typically bellicose language has shocked many South Koreans, who had thought the main target of the North's nuclear program was the United States.

Adding to South Koreans' worries, the North and its nuclear arsenal are in the hands of a young new leader, Kim Jong-un, whose brinkmanship appears to be an effort to ensure the support of his nation's powerful military.

The South also has a new president, Park Geun-hye, the daughter of a military strongman who stood firm against North Korea, who also faces pressure to stand fast against the North. Just two weeks after her inauguration, Ms. Park faces a crisis as the North makes vague threats interpreted by many South Koreans as the precursor to some sort of limited, conventional military provocation. Ms. Park has promised to retaliate if her nation is attacked, aware of the public anger directed at her predecessor, Lee Myung-bak, when he showed restraint after the North shelled a South Korean island in 2010, killing four people.

That kind of limited skirmish is a more likely threat than a nuclear attack, but such an episode could quickly inflame tensions and escalate out of control. Over the years, North Korea has sent armed spies across the border, dug invasion tunnels under it and infiltrated South Korean waters with submarines.

But beyond the immediate fear of a military provocation, analysts say deeper anxieties are also at work in the South. One of the biggest is the creeping resurgence of old fears about the reliability of this nation's longtime protector, the United States. Experts say the talk of South Korea's acquiring nuclear weapons is an oblique way to voice the fears of a small but growing number of South Koreans that the United States, either because of budget cuts or a lack of will, may one day no longer act as the South's ultimate insurance policy.

"The Americans don't feel the North Korean nuclear weapons as a direct threat," said Chung Mong-joon, a son of the founder of the Hyundai industrial group and the former leader of the governing party who has been the leading proponent of South Korea's development of a nuclear weapons program. "At a time of crisis, we are not 100 percent sure whether the Americans will cover us with its nuclear umbrella."

The United States, which still has 28,500 troops based in South Korea, has sought to assure its ally that it remains committed to the region as part of the Obama administration's strategic "pivot" to Asia. But analysts say the fact that senior leaders like Mr. Chung and a handful of influential newspaper columnists now call for the need for "nuclear deterrence," or at least hint at it, reflects widespread frustrations over the inability of the United States and other nations to end North Korea's nuclear weapons program. Until recently the idea was too radical for most mainstream leaders and opinion makers, including both deeply pro-American conservatives and nationalistic yet antinuclear liberals.

Advocacy for a nuclear-armed South Korea has been virtually taboo since the early 1970s, when the country's military dictator, Park Chung-hee, made a serious bid to develop a nuclear weapon, fearing that the United States might pull out of Asia after its defeat in Vietnam. After catching wind of the program, Washington forced Mr. Park, the new president's father, to stop, persuading him instead to rely on the United States, an agreement that has held ever since.

Mr. Chung and others say that if the United States does not allow South Korea to develop its own nuclear arms, it should at least restore the nuclear balance on the Korean Peninsula by reintroducing American atomic weapons, which were removed from bases in the South in 1991 in a post-cold-war effort to reduce tensions.

While such views remain extreme in South Korea, they underscore the fear here that the North may never give up its nuclear weapons. The South's new level of anxiety is also apparent in the widespread speculation here about when and where the North might carry out another, non-nuclear military provocation.

North Korea has stoked those fears by saying that on Monday it will drop out of the 60-year-old armistice that ended the Korean War, in a show of anger at new United Nations sanctions for its nuclear test. North Korea has threatened to terminate the armistice in the past, but the greater fear now is that it might take actions to contravene it. There have been cryptic warnings in North Korea's state-run news media of coming "counteractions," which have led South Korean officials to warn of an episode like the bombardment of Yeonpyeong Island in 2010.

On Friday, North Korea's state-run television showed Mr. Kim addressing the same artillery units that hit Yeonpyeong. On the same day, South Korean television stations showed President Park with heavily decorated generals, and later descending into the bunker at the Blue House, South Korea's version of the White House, to confer with her national security advisers.

The opposition parties had blocked the confirmation of her cabinet, raising concerns about her ability to respond to a crisis, but she reached a deal allowing her to fill crucial posts on Monday. Even many on the left said that the country would quickly pull together if shots were fired.

"The third test was a wake-up call for the left, too," said Lee Kang-yun, a television commentator.

On the streets of Seoul, it has remained business as usual with no signs of panic, a testimony to the resilience, or perhaps resignation, of a people who have grown used to the North's threats.

Chung Eun-jin, a 26-year-old English teacher interviewed in the trendy Gangnam district, said she was not overly concerned because the North had threatened the South so often before. But Kwon Gi-yoon, 38, an engineer, said that since the North's third test, he believed that South Korea should develop its own nuclear weapons.

Opinions like Mr. Kwon's appear to be spreading. Two opinion polls conducted after the third test, one by Gallup Korea and the other by the Asan Institute for Policy Studies, found that 64 to 66.5 percent of the respondents supported the idea that South Korea should develop its own nuclear weapons, similar to polls after the Yeonpyeong attack in 2010.

"Having a nuclear North Korea is like facing a person holding a gun with just your bare hands," said Mr. Kwon, the engineer. South Koreans should have "our own nuclear capabilities, in case the U.S. pulls out like it did in Vietnam."