American B-1B bombers made another “show of force” flight over South Korea on Tuesday, escorted by F-15 fighters from the South Korean military.

“The U.S. military said in a separate statement it conducted drills with Japanese fighters after the exercise with South Korea, making it the first time U.S. bombers have conducted training with fighters from both Japan and South Korea at night,” Reuters reports.

“Flying and training at night with our allies in a safe, effective manner is an important capability shared between the U.S., Japan and the Republic of Korea and hones the tactical prowess of each nation’s’ aviators. This is a clear demonstration of our ability to conduct seamless operations with all of our allies anytime anywhere,” said Major Patrick Applegate of the U.S. Air Force 613th Air Operation Center, as quoted by ABC News.

The South Korean military said the joint exercise demonstrated “the strong ability of the alliance against the North Korean nuclear missile threat.”

The bombers flew out of Andersen Air Force Base on Guam. North Korea’s periodical threats to attack Guam generally include complaints about the B-1B bombers based there.

In late September, North Korea threatened to shoot down American strategic bombers that come within range of their weapons, even if the planes are flying outside North Korean airspace. South Korean intelligence officials believe North Korea may have great difficulty detecting the presence of the stealthy American aircraft, in part due to technical difficulties with its radar installations. This has prompted concerns that paranoid North Korea might fire on unidentified targets if they believe B-1Bs are in the vicinity.

CNN notes that the aircraft carrier USS Ronald Reagan is scheduled to conduct exercises with the South Korean Navy at the end of October.

Also on Tuesday, Secretary of Defense James Mattis and Joint Chiefs Chairman Gen. Joseph Dunford announced that President Donald Trump received a briefing on “a range of options to respond to any form of North Korean aggression or, if necessary, to prevent North Korea from threatening the United States and its allies with nuclear weapons.”