The two Koreas traded hundreds of rounds of live artillery fire across their disputed maritime border Monday, forcing South Korean islanders to take shelter a day after the North drove up tensions by threatening a new nuclear test.

The exchange, triggered by a three-hour North Korean live-fire exercise that dropped shells into South Korean waters, was limited to untargeted shelling into the sea, military officials said.

South Korea's defence ministry said the North fired some 500 shells during the drill, around 100 of them landing on the south side of the sea boundary.

The ministry said the South had responded to Pyongyang's "premeditated provocation" by firing 300 shells from K-9 self-propelled howitzer batteries based on its front-line islands.

"If the North takes issue with our legitimate returning of fire and uses it to make yet another provocation towards our sea and islands, we will make a resolute retaliation," ministry spokesman Kim Min-Seok said.

The United States denounced North Korea's artillery fire, with the White House and the Pentagon accusing Pyongyang of "dangerous" behavior.

"The provocation that the North Koreans have once again engaged in is dangerous and it needs to stop," Pentagon chief Chuck Hagel told reporters in Washington.

Hagel said the North's actions would be "a subject that I will discuss with my counterpart in China" during a tour of Asia over the next two weeks.

Analysts said the incident, coming a day after Pyongyang threatened to conduct a "new" type of nuclear test, was largely a sign of the North's growing frustration with US resistance to resuming multi-party talks on its nuclear programme.

"I don't see that this ran any real risk of escalating into a serious clash," said Yang Moo-Jin, a professor at the University of North Korean Studies in Seoul.

"It's really North Korea showing it intends to keep the pressure on to resume a dialogue," Yang said.

Pyongyang sees the nuclear negotiations as an opportunity to win material concessions and aid from the international community.

The South Korean stock market shrugged off the incident, with the main Kospi index closing up 0.23 percent at 1,985.61.

- Unusual advance warning -

The North had ensured maximum publicity for its live-fire drill by taking the unusual step of notifying the South beforehand, and issuing a provocative no-sail, no-fly advisory.

The exercise began at 12:15pm (0315 GMT) and South Korea, which had threatened to respond if any shells crossed the border, retaliated shortly afterwards, the defence ministry said.

As a precaution, border island residents were evacuated to shelters, as South Korean fighter jets flew overhead. The evacuation order was lifted an hour after the North ended its drill.

In November 2010, North Korea shelled Yeonpyeong island just south of the sea boundary, killing four people and triggering concerns of a full-scale conflict.

China, the North's key ally, expressed concern and urged the two Koreas to exercise restraint.

"Currently there are raised tensions on the Korean peninsula, and we are concerned about this. We hope relevant parties exercise restraint," Chinese foreign ministry spokesman Hong Lei told reporters.

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.

Monday's incident coincided with a massive, amphibious landing drill by nearly 15,000 South Korean and US troops.

Last week, the North upped the ante by test-firing two mid-range ballistic missiles capable of striking Japan.

The UN Security Council condemned the launches, and Pyongyang responded with its threat of a new type of nuclear test -- a possible reference to testing a uranium-based device or a miniaturised warhead small enough to fit on a ballistic missile.

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.

- 'Severe cost' for nuclear test -

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 UN sanctions.

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