Mr. Trump added to the worries on Tuesday with Twitter messages declaring that if the Chinese would not help, “we will solve the problem without them!”
Although officials in South Korea have said the United States would never attack the North without first consulting the South Korean government, the confluence of events has revived fears of just such a possibility.
“The United States makes it clear that it will not take a new policy or measure without consultations with us,” Cho June-hyuck, a spokesman of the South Korean Foreign Ministry, said on Tuesday when asked by reporters about the rumors.
Moon Sang-gyun, a Defense Ministry spokesman, also warned that South Koreans should not be “deluded” by unfounded rumors spreading online.
Moon Jae-in, a leader of the main opposition Democratic Party, in a Facebook post that was widely cited in South Korean news media on Tuesday, issued his own warning against the possibility of an American pre-emptive strike carried out unilaterally.
“The safety of South Korea is as important as that of the United States,” he said. “There should never be a pre-emptive strike without South Korean consent.”
Mr. Moon is one of the two leading contenders, along with another opposition leader, Ahn Cheol-soo, for the May 9 presidential election to choose the successor to former President Park Geun-hye. Ms. Park, who was impeached by Parliament in December, was formally ousted in a Constitutional Court ruling in March. Prime Minister Hwang Kyo-ahn is serving as an acting president.
“Neighboring countries are taking advantage of the absence of a president in South Korea to try to exclude us and handle issues on the Korean Peninsula according to their own understanding,” Mr. Moon said, without blaming the Trump administration by name.
Although South Koreans generally consider the United States their most important ally in deterring North Korea, they remain deeply wary of any American attempts that they fear will raise tensions and even rekindle war. The Korean Peninsula remains technically at war because the 1950-53 Korean War was halted by an armistice, not a peace treaty.
The United States and South Korea would together win any war against North Korea, but many fear catastrophic devastation to South Korea, especially Seoul. The capital city — home to 10 million people and surrounded by satellite cities with another 10 million residents — lies within the range of North Korean artillery, rockets and short-range missiles amassed along the border.
In 1994, not long after the nuclear crisis first flared with North Korea, President Bill Clinton considered launching a surgical attack on the North’s main nuclear complex in Yongbyon north of Pyongyang, the capital, according to the South Korean president at the time, Kim Young-sam. Panic swept through South Korea, with people stocking up on food. Mr. Kim later said he had personally protested to Mr. Clinton, persuading Washington to drop the plan to strike Yongbyon.
After conducting five nuclear tests since 2006, North Korea is now widely believed to have several to as many as a dozen nuclear weapons, making a pre-emptive strike far riskier than it was in 1994. The North is also one of the world’s largest owners of chemical and biological weapons and apparently has no qualms in using them, as in the Feb. 13 assassination in Malaysia of Kim Jong-nam, the estranged half brother of the North Korean leader Kim Jong-un. Malaysia has said Kim Jong-nam was killed by the nerve agent VX.
Since Kim Jong-un took power five years ago, North Korea has conducted three nuclear tests and nearly 50 ballistic missile tests. And in his New Year’s Day speech, Mr. Kim said his country was in the “final stage” of preparing for its first test of an intercontinental ballistic missile.
Mr. Trump retorted at the time: “It won’t happen!”
Defense analysts said there were too many constraints for the United States to launch a pre-emptive strike against North Korea without expecting a major retaliation. North Korea keeps most of its crucial military assets in tunnels, and it remained unclear whether the United States and South Korean military planners had located them all. There are also 200,000 American civilians, as well as 28,500 United States troops, living in South Korea, and their neighborhoods would probably be among the first targets of any North Korean retaliation.
On Tuesday, North Korea’s main state-run newspaper, Rodong Sinmun, said that if the United States tried a pre-emptive strike, it would be “as foolish as putting its own neck on the chopping block.”
In another sign that North Korea has no intention of forsaking nuclear arms, top North Korean officials in Pyongyang gathered on Tuesday, the fifth anniversary of Mr. Kim’s election as leader of the ruling Workers’ Party, and vowed to uphold his policy of strengthening the North’s nuclear arsenal, state news media reported. Also on Tuesday, Ri Su-yong, an 82-year-old Politburo member, former foreign minister, and adviser and childhood mentor to Mr. Kim, was given another powerful position in Parliament. Last year, Mr. Ri told Chinese officials that North Korea’s nuclear weapons expansion was “permanent.”
With less than one month left before the presidential election in the South, how to deal with North Korea has become a dominant campaign issue.
Mr. Ahn, the presidential candidate of the centrist People’s Party, has recently surged in popularity to rival Mr. Moon in a neck-and-neck race. Many conservative voters who were disappointed by Ms. Park’s conservative camp were supporting Mr. Ahn, whom they considered tougher on North Korea than Mr. Moon, political analysts said.
Mr. Ahn said that if elected, he would honor the contentious decision by Ms. Park’s government to allow the United States to deploy an advanced antimissile defense system in the South.
China has vehemently opposed the deployment, calling it a threat to its own security, and South Korean shops, movies and TV dramas have been boycotted by many Chinese in recent months.
Mr. Moon, who has said South Korea should learn to “say no to the Americans,” vowed to review the deployment if elected.
On Tuesday, Mr. Moon said if North Korea froze its nuclear weapons program and returned to negotiations, the deployment of the so-called Terminal High Altitude Area Defense, or Thaad, could be suspended. But if North Korea conducts its sixth nuclear test and advances its nuclear weapons program, he warned, the deployment will become “inevitable.”