It remains unclear whether North Korea will lash out with a weapons test during the exercise, as it often has in the past.
On Friday, a researcher at the Institute for American Studies at the North Korean Foreign Ministry warned that the joint exercise, as well as a flight by two American B-1B bombers over South Korea on Tuesday, compelled the North to “take military counteraction.”
The researcher, Kim Kwang-hak, did not elaborate but recalled North Korea’s August warning that it could launch missiles near Guam, home to the United States air base from which the B-1B long-range bombers took off on Tuesday. Kim Jong-un, the North Korean leader, has said he would watch the Americans before deciding when to launch an “enveloping fire” around Guam.
“We have already warned several times that we will take counteractions for self-defense, including a salvo of missiles into waters near the U.S. territory of Guam,” Mr. Kim, the North Korean researcher, told the North’s official Korean Central News Agency on Friday. “The U.S. military action hardens our determination that the U.S. should be tamed with fire and lets us take our hand closer to the ‘trigger’ for taking the toughest countermeasure.”
North Korea has made similar threats against the United States for decades. But Mr. Trump has added to tensions in recent weeks by employing similarly tough talk, threatening to “totally destroy” or rain down “fire and fury” on North Korea. He has said Secretary of State Rex W. Tillerson was “wasting his time” trying to negotiate with North Korea.
Despite Mr. Trump’s tough talk, John Kelly, the White House chief of staff, said on Thursday that North Korea’s nuclear threat was “manageable” for now.
Mr. Kelly added that Americans should be concerned that the North is getting closer to achieving the ability to hit the mainland United States with its missiles. He said there was already “great concern” about Americans living in Guam.
“Right now we think the threat is manageable,” Mr. Kelly told reporters at the White House. “Let’s hope that diplomacy works.”
Also on Friday, South Korea’s meteorological authorities said that they detected a small quake near the North’s underground nuclear test site, but that it was not caused by a man-made explosion. They have detected three similar tremors from near the test site since the North’s nuclear test on Sept. 3, in which North Korea said it detonated a hydrogen bomb.
Some earthquake experts have attributed the recent tremors to underground cave-ins caused by that powerful test. Commercial satellite images have also found evidence of landslides near the North Korean site, raising fears of radioactive fallout if the North conducts another nuclear test there.
The previous test compelled Washington to accelerate its global campaign to exert sanctions and pressure on North Korea.
On Thursday, the United Arab Emirates said it would stop issuing new visas to North Korean workers. Kuwait and Qatar have taken similar steps in recent weeks. Several thousand North Korean workers have been working in Middle East construction sites, earning badly needed cash for their government.