North Korea staged an artillery firing drill on Friday near the South Korean border and warned that planned US-South Korean naval exercises at the weekend would bring the peninsula closer to war.
The shelling sent residents of Yeonpyeong island, targeted in a deadly North Korean bombardment on Tuesday, scrambling for air raid shelters, but Seoul said it appeared to be an exercise and that no shells landed on its territory.
The latest incident came as Seoul grappled with the domestic political fallout of Tuesday's attack after the defence minister resigned over criticism that the South's response to the bombardment had been too weak.
A defence ministry spokesman told AFP explosions were heard several times between noon and 3 pm (0300-0600 GMT) and appeared to have come from the North.
"We assume North Korea carried out firing exercises," he said.
A military source quoted by YTN television said there were about 20 shells which apparently landed on the North's side.
A YTN correspondent on Yeonpyeong island, which was hit by the bombardment Tuesday, said those residents who have not already fled for the mainland were rushing to air raid shelters.
Tuesday's shelling killed four people, injured 18 and set some 20 buildings and forests ablaze.
A US aircraft carrier battle group was heading for the yellow sea for the four-day drill starting Sunday, a show of force designed to deter the North after Tuesday's strike rang alarm bells worldwide.
The North, unrepentant over its earlier barrage, criticised "the US imperialists and south Korean puppet war-like forces" for what it called an exercise in "sabre-rattling".
"The situation on the Korean peninsula is inching closer to the brink of war due to the reckless plan of those trigger-happy elements to stage again the war exercises targeted against the (North)," its official news agency said.
Pyongyang has used such language frequently in the past. It was unclear whether it would take any measures to try to disrupt the drill — which has also been criticised by the North's sole major ally China.
The US military says the exercise is defensive and planned well before the North's "unprovoked artillery attack" but it demonstrates the US "commitment to regional stability through deterrence".