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".
The North has said its firing Tuesday was retaliation, after South Korea had fired shells into its waters during an exercise. But South Korea expressed outrage at the first bombardment of its civilian areas since the 1950-53 war.
However its response to the attack was widely criticised as too soft and Defence Minister Kim Tae-Young resigned Thursday. The presidency said it was still screening candidates to replace Kim.
The South fired 80 shells at the North's coastal artillery positions Tuesday in response to the barrage, but says it did not call in air strikes to avoid escalating the situation.
Seoul military officials said that their firing had started fires and caved in trenches in the North. "We suspect the North also sustained considerable damage," one official said.
The South plans to send more troops and guns to five frontline islands and also said it would change the rules of engagement to let its military hit back harder in case of future attacks.
The North, in another statement Friday, vowed to stage such attacks if there is any further "provocation" by the South.
Accusing the South of confrontational tactics, it said the North is "getting fully ready to give a shower of dreadful fire and blow up the bulwark of the enemies if they dare to encroach again upon the (North's) dignity and sovereignty even in the least".
South Korea, the United States, Australia and other nations have pressed China -- the North's economic lifeline -- to rein in its unruly neighbour.
China has criticised plans for the US-South Korean war games but has failed to join international condemnation of the North's bombardment.
Parts of Yeonpyeong island resembled a war zone Friday, an AFP correspondent reported. Stray dogs in the streets begged for food after their owners joined an exodus of hundreds of villagers to the mainland.
Charred ruined buildings could be seen along streets littered with glass and household items and draped with loose power lines. A liquor store displayed hundreds of shattered bottles.
Some residents fled the island for fear of more shelling from the North when the joint naval exercise starts, said policeman Kang Sang-Beom.
"My mission is to prevent possible looting as the village is virtually deserted," Kang said. "We are also looking for any more dead."
World powers are struggling to draw up a response to the latest actions by a regime that has in recent years staged two nuclear tests and fired long-range missiles.
It is also accused of sinking a South Korean warship in March with the loss of 46 lives, a charge it denies.
Many observers believe the attack was meant to highlight the military credentials of heir apparent Kim Jong-Un, youngest son of leader Kim Jong-Il.