The powerful sister of North Korean leader Kim Jong-un has blasted South Korea and the United States for conducting a joint military exercise. 

Kim Yo-jong issued the criticism in a statement on Tuesday as Seoul and Washington kicked off preliminary training ahead of next week's main summertime military exercise. 

Kim said in the statement that South Korea and the U.S. started the drill that further escalates instability and that the North expresses "strong regret over the act of betrayal by South Korea."

Calling the exercise the expression of the U.S.' hostile policy against North Korea, Kim warned that the dangerous war rehearsal by South Korea and the U.S. in disregard of the North's repeated warnings will lead them to face a grave security threat. 

She also said the Biden administration's North Korea policy of the so-called diplomatic engagement and dialogue without preconditions is nothing but hypocrisy and a disguise to hide its ulterior ambition of invasion. 

Kim then added the North will continue to enhance its capabilities of national defense and preemptive attack to swiftly respond to any military action.