North Korea announced a live-fire drill near its maritime border with South Korea on Tuesday, prompting Seoul to vow a "strong" response after a similar exercise a month ago triggered an artillery exchange.
"Our military is fully prepared," defence ministry spokesman Kim Min-seok said after Pyongyang notified Seoul of its proposed drill.
"If any shell lands on our side of the border, South Korea will respond strongly," Kim told reporters.
South Korean fishing vessels were told to stay away from the area and local officials on the border islands of Yeonpyeong and Baengnyeong said residents were advised to take precautions.
"Loudspeakers are relaying the message for residents to prepare to enter shelters whenever necessary," a local official on Baengnyeong told AFP.
North Korea carried out a similar drill on March 31 during which a number of shells dropped into South Korean waters.
The South responded in kind, and the two rivals traded hundreds of rounds of live artillery fire, forcing islanders to take shelter.
The exchange was limited to untargeted shelling into the sea, but fuelled tensions that had already risen after North Korea threatened to carry out a nuclear test.
Tuesday's announcement followed analysis of recent satellite images suggesting the North was indeed preparing to conduct its fourth atomic detonation, with stepped-up activity detected at its main nuclear test site.
It also followed the visit to Seoul last week by US President Barack Obama, who angered Pyongyang by demanding that the North abandon its nuclear weapons programme and by threatening tougher sanctions if it went ahead with another test.
"North Korea's continued pursuit of nuclear weapons is a path that leads only to more isolation," Obama told American troops based in Seoul.
"It's not a sign of strength. Anybody can make threats. Anyone can move an army. Anyone can show off a missile," he said.
North Korea denounced Obama's visit as provocative and said it had only reaffirmed Pyongyang's policy of preparing to fight "a full-scale nuclear war".
North Korea has conducted three nuclear tests, the most recent -- and most powerful -- in February last year.
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 the US-led United Nations forces after the 1950-53 Korean War.
Both sides complain of frequent incursions by the other and there were limited naval clashes in 1999, 2002 and 2009.
In November 2010, North Korea shelled the South Korean border island of Yeonpyeong, killing four people and briefly triggering concerns of a full-scale conflict.
Border island residents have become used to the North's provocative behaviour and some just brushed off Tuesday's live-fire alert.
"I'm so sick of it," said one inn owner on Baengnyeong island.
"The whole thing is just scaring my customers away. I'm more likely to die from business losses than an attack from the North," she told AFP by telephone.
Inter-Korean ties had seemed to be enjoying a thaw earlier this year when the North -- following rare, high-level official talks -- hosted the first reunion for more than three years of families separated by the Korean War.
But tensions began to escalate after the South launched its annual joint military exercises with the United States in late February.
The North was also angered by a UN report detailing Pyongyang's record of systematic human rights abuses and by the UN Security Council's criticism after it test-fired two medium-range missiles in March.
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