PYONGYANG, North Korea — North Korean leader Kim Jong Un on Friday declared his front-line troops in a “quasi-state of war” and ordered them to prepare for battle a day after the most serious confrontation between the rivals in years.

South Korea’s military on Thursday fired dozens of artillery rounds across the border in response to what Seoul said were North Korean artillery strikes meant to back up a threat to attack loudspeakers broadcasting anti-Pyongyang propaganda.

The spike in tensions prompted the U.S. and South Korea to briefly halt an annual military exercise that began this week, U.S. defense officials said Friday.

The North’s declaration Friday is similar to its other warlike rhetoric in recent years, including repeated threats to reduce Seoul to a “sea of fire.”

Still, the North’s apparent willingness to test Seoul with military strikes and its recent warning of further action raise worries because South Korea has vowed to hit back with overwhelming strength should North Korea attack again.