North Korea announced a live-fire drill near its disputed maritime border with South Korea, further ratcheting up tensions a day after threatening a "new form" of nuclear test.
There was a swift response from the South Korean military, with the Joint Chiefs of Staff warning of immediate retaliation if any ordinance is fired across the border.
The precise nature of the exercise was unclear, but the Yellow Sea border is an extremely sensitive region that has been the scene of brief but bloody clashes in the past.
In November, 2010, North Korea shelled a South Korean island near the border, killing four people and triggering concerns of a full-scale conflict.
It is not unusual for North Korea to carry out a live-fire exercise, but it does not normally take the precaution of notifying the South in advance.
"The fact that they have sent such a message to us indicates their hostile intention," said South Korean Defence Ministry spokesman Wi Yong-Seop.
"The aim is to threaten us and rack up tension on the Yellow Sea border and the overall Korean peninsula," Wi said.
The North's notification designated seven areas close to the border and said all South Korean vessels should be kept away from them.
"We notified the North that we would strongly respond with fire if it fires across the border," a South Korean military official said.
Pyongyang has carried out a series of rocket and short-range missile launches in recent weeks, in a pointed protest at ongoing annual South Korea-US military exercises.
On Wednesday it upped the ante by test-firing two mid-range ballistic missiles capable of striking Japan.
It was the first medium-range missile launch since 2009 and coincided with a trilateral summit attended by the South, the United States and Japan that focused on presenting a united front to the dangers posed by Pyongyang's nuclear ambitions.