"Satellite images show our shells landed on a cluster of barracks in North Korea, so we presume there have been many casualties and considerable property damage," a senior Joint Chiefs of Staff official told the South's Yonhap News Agency in the military's first mention of human casualties in North Korea.
Seoul is still analyzing satellite images to assess the extent of North Korean damage and casualties, the unidentified official told the news agency.
He declined to elaborate further.
Earlier in the day, a South Korean lawmaker suggested the Southern retaliation probably produced "severe human casualties" in North Korea, based on satellite images showing the counter-fire by South Korea's military hit one of the North's barracks near the tense Yellow Sea border.
"About 10 artillery shells fired by our military landed onto a military unit compound on (North Korea's) Mu-do (island) and one of them directly hit a barrack," said Rep. Kwon Young-se of the ruling conservative Grand National Party, citing two satellite images from the National Intelligence Service, the nation's spy agency.
"There might have been severe human casualties," Kwon told reporters after a parliamentary committee meeting with NIS officials.
Mu-do is a small island north of the Yellow Sea border near where a North Korean artillery battalion is believed to have launched the attack.
North Korea's artillery attack Nov. 23 killed two South Korean marines and two civilians on the South Korean island of Yeonpyeong, close to the countries' disputed maritime border.
The barrages from the North, which claimed it was provoked by artillery fire by the South, were the first attack on a civilian area since the end of the 1950-1953 Korean War.