SEOUL – North Korea on Thursday threatened unspecified retaliation after the U.S. flew long-range bombers over South Korea during training with South Korean warplanes, which North Korea views as practice for an attack against it.

The U.S. flew the B-1B bombers Tuesday to train with other U.S. and South Korean fighter jets. South Korea’s Defense Ministry had said the drill was meant to show the two countries’ combined deterrence capability against North Korea’s advancing nuclear program.

The U.S. and South Korea routinely hold joint military exercises that they describe as defensive in nature. But North Korea regards them as an invasion rehearsal and is particularly sensitive to the U.S. mobilization of strategic assets such as long-range bombers, aircraft carriers and nuclear-powered submarines.

“The recent military move of the U.S. and the ROK is an open threat to the security of our state and a grave provocation that raises the military tension in the region to an extreme dangerous level,” an unidentified spokesperson for North Korea’s Defense Ministry said in a statement carried in state media.

ROK stands for the Republic of Korea, South Korea’s formal name.

The statement warned that North Korea “will deter by dint of powerful force the U.S. aggressive attempt to permanently fix the malignant instability element in the security environment of the region.”

North Korea often test-launches missiles in response to the U.S. flyovers of B-1B bombers, which is capable of carrying a huge payload of conventional weapons.

In a development that could further anger North Korea, South Korea’s air force announced later Thursday that it was beginning its large-scale biannual aerial exercise with U.S. forces from Thursday. The air force said the two-week “Freedom Flag” exercise would involve 90 aircraft and other aerial assets from South Korea and the U.S.

Animosities on the Korean Peninsula are running high as North Korea continues weapons tests designed to modernize his nuclear arsenal and supports Russia’s war against Ukraine by supplying weapons and troops.

Since his inauguration, President Donald Trump has repeatedly boasted of his personal ties with Kim and expressed his willingness to reach out to North Korean leader Kim Jong Un to revive diplomacy.

On March 31, Trump called Kim “a very smart guy” and North Korea “a big nuclear nation.” Trump said he and Kim “have a great relationship” and that “there is communication,” though there are no known public negotiations and North Korea hasn’t publicly responded to Trump’s outreach.