North Korea has begun conducting live-fire artillery drills near the disputed inter-Korean western maritime border, South Korean officials say.
The drills immediately invited a stern rebuke from South Korea who warned that they would "respond with counter fire" if the North launched fire towards the maritime border.
"The South Korean Army, Air Force and Navy is fully prepared for possible North Korean provocation," said Kim Min-seok, a South Korean defence ministry spokesman.
Both Koreas regularly carry out drills near South Korean islands and the North Korean mainland in the Yellow Sea. But they can be sensitive because of the disputed maritime line separating the countries.
Pyongyang had notified Seoul that it would be conducting the exercises. They are seen as a possible indication of rising frustration in Pyongyang as it unsuccessfully pushes for outside aid.
A Defence Ministry official, speaking on condition of anonymity because of office rules, confirmed that the drills had begun but provided no other details.
Last week South Korean officials claimed that the North had completed the steps required to conduct a nuclear test, including the sealing of a tunnel at the Punggye-ri site in the north east of the country.
The so-called Northern Limit Line was drawn by the US-led UN Command without Pyongyang's consent at the close of the 1950-53 Korean War, which ended with a truce, not a peace treaty, leaving the peninsula still technically in a state of war. North Korea routinely argues that the line should run farther south.
A year after threatening each other with war, the Koreas had restored some trust and held reunions earlier this year of families divided by the war. But ties have since steadily soured. Pyongyang this week launched a sexist rhetorical attack on President Park Geun-hye, calling her to a "despicable prostitute."
North Korea test-fired two medium-range ballistic missiles in March into the Sea of Japan. Pyongyang had earlier staged a series of shorter-range rocket launches to protest annual springtime military drills by the US and South Korea that Pyongyang says are invasion preparation.
South Korean officials have also warned the North could be preparing for its fourth nuclear test.
The North's live-fire drills come as South Korea deals with the tragedy of a ferry sinking that has left more than 300 dead or missing.
Additional reporting by AP