REUTERS

South Korean Vice Defense Minister Baek Seung-joo said it was likely the North would fire at some of the 11 sites where the loudspeakers are set up on the South's side of the Demilitarized Zone, which separates the two countries, according to Reuters.

Tension escalated on Thursday when North Korea fired shells into South Korea to protest against the broadcasts. The South responded with its own artillery barrage. Both sides said there were no casualties or damage in their territory.

The North's shelling came after it had demanded last weekend that South Korea end the broadcasts or face military action - a relatively rare case of it following up on its frequent threats against the South.

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Read alsoSouth Korea worried over reports Pyongyang can launch missiles from submarinesIts 48-hour ultimatum to halt the broadcasts, delivered in a letter to the South Korean Defense Ministry via a joint military communications channel, was also uncharacteristically specific. The deadline is around 17:00 (0400 EDT) on Saturday in Seoul.

The U.S. military, which has 28,500 personnel based in South Korea, said it was closely monitoring the situation.

Washington earlier urged Pyongyang to halt any "provocative" actions in the wake of Thursday's exchange of fire, the first between the two Koreas since last October. Japan called on North Korea to exercise restraint.