Civilians on five South Korean border islands were ordered into bomb shelters on Monday, amid fears that North Korea would retaliate against a live-fire military exercise in a disputed border region.
Four people were killed when the North shelled Yeongpyeong island during an ewercise last month and the North had threatened an even deadlier response this time, saying the shells fired in such drills fall into its territorial waters.
The ABC quotes a South Korean defence ministry official as saying jet fighters were on airborne alert during the firing, and the Yonhap news agency as saying two destroyers were deployed in forward positions in the Yellow Sea.
Meanwhile, the BBC reports, a United Nations Security Council meeting on the Korean crisis has broken up without agreement.
Speaking to reporters after eight hours of discussions, Russian ambassador Vitaly Churkin called on South Korea to cancel its plans.
"It's better to refrain from doing this exercise at this point in time," he said.
Separately, CNN is reporting North Korea has offered to allow UN nuclear inspections to resume. It says the offer followed talks in Pyongyang with the United States intermediary Bill Richardson.
CNN has a team travelling with the New Mexico governor.