South Korea has returned fire into North Korean waters after the North conducted live-fire drills near disputed sea boundary.
Officials on the South Korean border islands of Yeonpyeong and Baengnyeong said residents were being evacuated to shelters as a precaution following the incident on Monday.
“Some of the shells fired by North Korea dropped in our area and our side responded with fire,” a spokesman for the South Korean Joint Chiefs of Staff (JCS) told AFP news agency.
There was no indication that either side was firing at any particular target.
Earlier on Monday, North Korea conducted a live-fire drill, further ratcheting up tensions a day after threatening a “new form” of nuclear test.
South Korea’s military had warned of immediate retaliation if any ordnance 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.
Pyongyang has carried out a series of rocket and short-range missile launches in recent weeks, in a pointed protest against 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 US and Japan that focused on presenting a united front to the dangers posed by Pyongyang’s nuclear ambitions.
In an interview with Al Jazeera, John Delury, a professor at Yonsei University in Seoul, said North Korea will likely continue doing its own test while the South Korea-US military exercises are going on.
“I don’t think that we have seen the end of this yet,” Delury said, adding that the North had even threatened on Sunday to conduct a fourth nuclear test.
UN resolutions prohibit the North from firing any ballistic missiles and the UN Security Council said it would consider taking “appropriate” action – a response that triggered the North’s “new” nuclear test threat.
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.