South Korean Defense Ministry officials said marines on the Yeonpyeong Island, 75 miles west of Seoul, detected three artillery shots from a North Korean island across their disputed maritime border, The New York Times reported.
One of the shells was believed to have landed on the so-called Northern Limit Line, a ministry spokesman said.
North Korea does not recognize the U.N.-drawn line at the end of the 1950-53 Korean War, insisting on a location south of it.
"We responded by broadcasting a warning and then firing three artillery shells on the N.L.L.," the South Korean spokesman said. "We are paying close attention to the situation, but so far we have detected no further unusual activities from the North."
Relations between the two Koreas have deteriorated since last year's sinking of a South Koran war ship that killed 46 sailors and that Seoul blamed on the North, and a similar shelling of the same island last November, which killed four people including two South Korean marines.
Also Wednesday, the Times said there were local reports South Koreans were on the lookout for an assassination squad allegedly assigned by North Korea to kill South Korean Defense Minister Kim Kwan-jin. Officials neither confirmed nor denied the reports.