North Korea has fired shells into the Yellow Sea near its border with the South, South Korean sources say.
A battery reportedly opened fire around 1730 local time (0830 GMT), directing shells into its own waters.
The firing came just after the end of South Korean-US naval exercises which had provoked warnings of retaliation from the North.
Tension between the two states rose this year after one of the South's warships was sunk.
The South accuses the North of sinking the Cheonan in March with the loss of 46 sailors.
Earlier, the UN reported that the North had agreed to a new meeting on the sinking of the ship.
Colonels are due to meet at the Panmunjom truce village, which straddles the two states' border, on Tuesday.
It will be the fourth round of such talks.
It is not the first time that North Korea has tested its artillery in this way, having sent a similar number of shells into the same waters in January, the BBC's John Sudworth reports from Seoul.
At the time, South Korea and the US reacted angrily.
There is unlikely to be any great alarm from the South Korean public, although perhaps some concern that the displays of military might could lead to a further, dangerous escalation, our correspondent adds.
The shelling appears to have begun about half an hour after the naval exercises ended.
"All we know is that North Korea fired artillery rounds into its sea off the west coast, but we have no more details as of now," Lt Cdr Yu Jae-il of the South's joint chiefs of command told Reuters news agency by phone from Seoul.
About 100 shells were apparently fired, falling harmlessly into the sea.
On Sunday, the South accused the North of detaining one of its fishing boats, the Daeseung.
The ship had stopped sending signals after a fishing trip to the Sea of Japan, known in Korea as the East Sea, on Saturday.
South Korea's coastguard said they had learnt that the ship was being investigated by North Korean officials.
They called for a "swift resolution to the matter", and safe return of the vessel and its crew.