The North Korean military warned on Wednesday that it would “burn up all the objects” in border regions of the South, including Seoul, “the moment the U.S. reckless attempt at pre-emptive attack is spotted” and that “the whole of the southern half” of Korea would be its target.

But North Korea has a multitude of options, especially ones in which its enemies would not quickly be able to trace the origin, like cyber attacks, analysts said.

It could also, for example, seize Japanese, South Korean and American citizens and hold them hostage, using them as a leverage to drive a wedge between Washington and its allies, especially should Mr. Trump launch a military attack without consulting American allies, they said.

North Korea could mobilize its sleeper spies in the South or use drones to attempt terrorist attacks, possibly involving chemical and biological agents, analysts said. In recent months, South Korea has found several North Korean drones flying into its airspace.

North Korea proved it had no qualms about using chemical weapons when its agents hired two female assassins to kill Kim Jong-nam, Mr. Kim’s estranged half brother, with the chemical agent VX in the Kuala Lumpur international airport in February.

“If a bunch of people start getting sick, how do you know for sure?” said Scott W. Harold, an associate director of the RAND Center for Asia Pacific Policy. “It’s all nightmare scenarios we talk about.”

Although South Korean news media gave prominent coverage of the latest escalation between North Korea and the United States, South Korea in general seemed little disturbed by either Mr. Trump’s “fire and fury” comment or the North’s threat to attack Guam. Most South Koreans appeared to consider the tit-for-tat just another episode in the decades-old ebb and flow of tensions and jitters they have grown used to.