South Korea has returned fire into North Korean waters after the North conducted live-fire drills near disputed sea boundary, with the White House and Kremlin expressing concern over North Korea's actions and rhetoric.
South Korea's defense ministry said the North fired about 500 shells during the drill on Monday, with around 100 shells landing on the south side of the sea boundary.
Ministry spokesman Kim Min-Seok said the South had responded to Pyongyang's "premeditated provocation" by firing 300 shells from K-9 self-propelled howitzer batteries based on its frontline islands.
Officials on the South Korean border islands of Yeonpyeong and Baengnyeong said residents were being evacuated to shelters as a precaution after the incident.
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 were 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.
White House national security spokesman Jonathan Lalley said North Korea's actions were "dangerous and provocative," while Russia's foreign ministry issued a statement saying it was "worried about a mutual toughening of rhetoric, including the declaration by North Korea that it could conduct a new nuclear test."
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.
North Korea has conducted three nuclear tests, the most recent — and most powerful — in February last year.
Most experts believe it is still some way from mastering the technology required to build a miniaturised warhead — a development that would be seen as a game-changer in assessing the North's nuclear arms capabilities.
South Korean Foreign Minister Yun Byung-Se warned Monday that the North would pay a "severe cost" if it went ahead with another test in defiance of existing UN sanctions.