North Korean forces fired over 100 artillery rounds into the Yellow Sea near the disputed border with the South on Monday, a media report said.
About 110 rounds were fired on North Korea's side of the maritime border, the Yonhap news agency reported citing South Korean military officials.
The firing followed five days of South Korean naval exercises near the sea border. North Korea had threatened "strong physical retaliations" to the manoeuvres, dpa reported.
The South and United States carried out the exercises in response to the sinking of one of South Korea's warships in March in which 46 sailors died. South Korea blamed its Stalinist neighbour for sinking the corvette, an accusation the North rejected.
US State Department spokesman PJ Crowley called Monday's episode another provocative act that most likely produced little more than "a lot of dead fish."
"All we can continue to communicate to North Korea is that there will be no reward for these provocations," he said.
The maritime border was set by the United Nations at the end of the 1950-53 Korean War, but North Korea does not recognize it.
Naval forces from South and North Korea clashed in the area in 1999, 2002 and 2009.