United State forces briefly suspended exercises in South Korea on Thursday after North Korean artillery fired several rounds across the border, a Pentagon spokesperson said on Friday.
Commander William Urban said US forces have since resumed scheduled joint exercises with the South Korean military, but remain in "an enhanced state of readiness."
"Such provocative actions heighten tensions and we call on Pyongyang to refrain from actions and rhetoric that threaten regional peace and security," Urban said.
Military tensions have soared on the divided peninsula since Thursday's exchange of artillery fire, which the south says was begun by the north.
North Korean leader Kim Jong-UN has put frontline troops on alert to back up his demand that South Korea halt propaganda broadcasts across the border, while the South Korean army was placed on maximum alert.
The North Korean foreign ministry warned Pyongyang was "prepared to risk an all-out war not just to simply respond or retaliate, but to defend the system our people chose."
"The situation has reached the verge of war and can no longer be reversed," it said. There are 28,000 US troops in South Korea, plus another 3,000 in country for the annual exercise.