SEOUL - North Korea announced a live-fire drill Monday near its disputed maritime border with South Korea, further ratcheting up tensions a day after threatening a "new form" of nuclear test.
The South's Yonhap news agency, citing an unnamed government official, said the exercise began around 12:15pm (0315 GMT), with artillery shells landing in North Korean waters, north of the South-controlled Baengnyeong island.
There was no immediate official confirmation that the drill was under way, but the Joint Chiefs of Staff (JCS) warned of immediate retaliation if any ordinance landed on the South side of the border.
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 notify 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, adding that Seoul was closely monitoring the situation.
'New form' of nuclear test
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 JCS official told reporters.
Monday's announcement came a day after North Korea threatened to carry out a "new form" of nuclear test - seen as a possible reference to efforts to build a nuclear warhead small enough to fit on a ballistic missile.
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.
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.
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.
The de-facto maritime boundary between the two Koreas - the Northern Limit Line - is not recognised by Pyongyang, which argues it was unilaterally drawn by US-led United Nations forces after the 1950-53 Korean War.
Both sides complain of frequent incursions by the other and these resulted in limited naval clashes in 1999, 2002 and 2009.
North rights record condemned
North-South tensions have been rising for weeks, undermining hopes raised after the North last month hosted the first reunion for more than three years of families separated by the 1950-53 war.
As well as the annual South Korean-US military drills, the North has been angered by efforts to bring Pyongyang before the UN Security Council over a UN report detailing Pyongyang's record of systematic human rights abuse.
On Friday, the UN Human Rights Council adopted a resolution urging the Security Council to ensure that those responsible for "gross human rights violations" in North Korea be held to account.
North Korea's foreign ministry called approval of the resolution a "vicious, hostile" act, engineered by the United States.