North Korea has begun evacuating some citizens into tunnels with emergency provisions and putting military camouflage on buses and trucks, the South Korean Defense Ministry said Tuesday.

The South Korean officials characterized the moves as feeding a war fever — a tactic that North Korean leaders have used in past times of tension, suggesting their country is under imminent threat in order to build support among their people.

The threats on both sides of the border have been intensifying in recent weeks, after the North’s third nuclear test and resulting international sanctions. The North has gone further than it has in the past — saying it could stage a pre-emptive nuclear strike on the United States and Seoul.

“All the enemies quite often playing with fire in the sensitive hot spot should be thrown into a caldron once I issue an order,” the North’s official Korean Central News Agency quoted its leader, Kim Jong-un, as saying Monday during a visit to a front-line artillery unit that faces South Korean marines on a nearby island.

“Once an order is issued, you should break the waists of the crazy enemies, totally cut their windpipes and thus clearly show them what a real war is like.” 

The South, under the new government of President Park Geun-hye — the South’s first female leader — has also issued tough statements, saying that if the North proceeded with nuclear attacks, its government would be “erased from the earth.”

On Tuesday, the South scoffed at the North’s announcement on Monday that the North had nullified the 1953 Korean War armistice, saying such unilateral action is prohibited under the terms of the cease-fire. Despite the bluster, there have been no indications that the North was using its declared end of the armistice to take any drastic steps.

Kim Min-seok, spokesman for the South Korean Defense Ministry, said Tuesday that there had been no sign of imminent nuclear or missile tests by the North or hostilities along the inter-Korean border. He said that the “rhetorical threats” flooding the North’s state-run news media had been aimed at putting “psychological pressure” on the South, which just began its annual Key Resolve joint military exercises with the United States, which are in addition to their two-month Foal Eagle drill that began on March 1.

“Through a series of political and military activities, North Korea is strengthening the solidarity of its people at home, while using the Key Resolve and Foal Eagle exercises as a pretext to threaten and pressure South Korea and the United States to change their policies,” Mr. Kim said. “If they launch a provocation, we will respond more strongly and make sure that they suffer far more.”

Stressing the alliance against North Korean threats, the office of the South Korean president announced Tuesday that she planned to meet President Obama in Washington in early May in her first such meeting since taking office on Feb. 25.

In Washington, Gen. C. Robert Kehler, the commander of United States Strategic Command, which oversees all American nuclear forces, told the Senate Armed Services Committee that “deterring North Korea from acting irrationally is our No. 1 priority.” He said that effort begins with the alliance with South Korea, but extends “to other forces that are available” in the Pacific and “ultimately, all the way back to our nuclear deterrent.”

“We are capable of offering to the president a full range of options,” General Kehler said, referring to the American options should the North initiate an attack. “Whatever he chooses to use in response to a North Korean act, I believe we can make available to him.”

But he said that the intelligence agencies and the military were examining whether “the limited missile defense that we have in place” in Alaska and California needed to be altered to keep pace with the North’s newly demonstrated capabilities.

Raising the fear of military clashes by threatening to scrap the armistice has long been part of North Korea’s efforts to draw Washington into negotiations. North Korea demands a formal peace treaty to end the Korean War, as well as security guarantees from the Americans. It also seeks direct talks with Washington for the perceived prestige it would bring the country.

As the crisis over North Korea’s nuclear ambition has dragged on, some analysts have argued that Washington should try negotiating a comprehensive deal that addresses North Korea’s concerns in return for the North’s giving up its nuclear weapons.

But South Korean and American officials also suspect that North Korea would turn talks for a peace treaty into endless haggling, demanding the removal of the 28,500 American troops stationed in South Korea and the reduction of American arms in the region.

In Vienna, a group that monitors nuclear testing said Tuesday that it had been unable to discern what type of fissile material had been used in the North Korean nuclear test. “It is very unlikely that we will register anything at this point,” said Annika Thunborg, a spokeswoman for the group, the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization, in an e-mail message.

The group’s monitoring system had detected a seismic disturbance from the test that registered a magnitude of 4.9 from 96 tracking stations, she said, but without access to the test site there was little else it could do.

Scientists had hoped detection of airborne particulates would help determine whether the North had used plutonium to fuel the test, as it had done earlier, or uranium. A switch to uranium could enable the North to increase its nuclear arsenal.

Had the test ban treaty been in force, Ms. Thunborg said, “on-site inspections would have been an option to search the location for evidence of a nuclear explosion.” “Without the treaty in force, we cannot make use of this option, unfortunately,” she added.

The treaty was negotiated between 1994 and 1996 but has not taken force because some nations have not signed it, including North Korea, while others have signed but not ratified it, including the United States and China.