The United States and South Korea held live-fire drills Monday near the demilitarized zone as tensions remain high with nuclear-armed North Korea.

North Korea has accused the US of engaging in a "hostile military act" and posing a "grave security threat" to the country by sending a nuclear submarine to a South Korean port. The USS Alexandria entered the Busan port on Monday for supplies and rest for its crew,

North Korea on Tuesday denounced the recent arrival of a U.S. Navy attack submarine in Busan, warning that it would respond with "corresponding strength" against what it views as U.S. military intimidation.