South Korean Marines fired warning shots Wednesday after a North Korean artillery shell landed off an island near the flashpoint Yellow Sea border, Seoul's defence ministry said.

The incident sparked a brief panic on the South's Yeonpyeong island, which was hit by a deadly North Korean barrage last November, but Seoul officials said the situation was now stable.

High tensions persist along the disputed western sea border, which has been the scene of bloody naval skirmishes in recent years.

The ministry said its troops heard the sound of three shells and saw one of them fall near the border known officially as the Northern Limit Line (NLL).

They were thought to have been fired by the North's coastal artillery, possibly during a training exercise.

In response Marines based on Yeonpyeong broadcast a warning and then fired three warning shots from K-9 self-propelled guns. These landed near the NLL.

"The situation is now stable," a ministry spokesman told AFP. "After we fired back, there was no further response from the North.

"We are still on alert but it seems that there will be no further provocative acts from North Korea at the moment."

Troops on Yeonpyeong and other frontline islands have been on high alert since last November's bombardment, which killed four South Koreans including civilians and damaged scores of buildings.

The government has reinforced troops and sent extra weaponry to the islands.

The firing briefly sparked alarm on Yeonpyeong, where some 1,800 civilians live along with the Marine garrison.

"The residents were preparing to evacuate their homes for shelters since they went through a similar thing in the past," a spokeswoman for Ongjin county, which oversees the island, told AFP.

"But they did not actually move to shelters since things have calmed down," the spokeswoman said, adding that the North's shells apparently landed on its own side of the NLL.

The incident came after the North made apparent peace overtures in recent weeks and expressed interest in restarting stalled six-nation nuclear disarmament talks.

Nuclear envoys from the two Koreas held rare talks in Bali last month, and a senior North Korean official visited New York later for discussions with US officials.

It was unclear whether the North's shells landed on the South's side of the border, which was drawn unilaterally by United Nations forces after the 1950-53 war.

The North refuses to accept it and says it should run further to the south.

"We're trying to establish whether the shots were fired intentionally or accidentally," an unidentified government official told Yonhap news agency. "I don't think it's something serious."

The boundary line was the scene of deadly naval clashes in 1999, 2002 and November 2009.

The South also accuses the North of torpedoing one of its warships near the NLL in March 2010, with the loss of 46 lives.

The North denied the charge but last November shelled Yeonpyeong in the first attack on a civilian-populated area in the South since the war.

The North said it was responding to a South Korean artillery drill which encroached into its waters.

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