(TIME) — Pyongyang has put its forces put on high alert after U.S. warships entered the South Korean port of Busan in preparation for a joint military exercise with Seoul and Tokyo.

On Monday Rodong Sinmun, the official newspaper of the Pyongyang regime, called the exercise a “bellicose attempt to escalate the situation on the Korean Peninsula […] by openly threatening it with nukes,” referring to the presence of the aircraft carrier USS George Washington. (The U.S. has a policy of neither confirming nor denying whether its ships are equipped with nuclear armaments.)

A North Korean military spokesman said that the U.S. would be “wholly accountable for the unexpected horrible disaster” that faced its “imperialist aggression forces.”

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