North Korea on Friday threatened a "merciless" military strike on South Korean territory next week, prompting a swift vow of retaliation from Seoul in a serious escalation of cross-border tensions.
The Korean People's Army (KPA) said it would launch the attack if North Korean defectors in the South went ahead with plans to scatter anti-Pyongyang propaganda leaflets Monday from balloons floated over the border.
The threat came a day after South Korean President Lee Myung-Bak made a surprise visit to an island close to the disputed maritime frontier that was shelled by the North two years ago.
"The moment a minor movement for the scattering is captured... a merciless military strike by the Western Front will be put into practice without warning," the KPA said in a statement carried by the official Korean Central News Agency.
"The surrounding area will become targets of direct firing of the KPA," the statement said, advising all local residents to "evacuate in anticipation".
A group of North Korean defectors plan to carry out the leafleting exercise on October 22 at 11:30 am (0230 GMT) at the border near the town of Paju, around 60 kilometers (35 miles) north of Seoul.
Such events are relatively common and North Korea has threatened action in the past, but Friday's statement was unusually strong with its specific naming of the time and location, coupled with the evacuation warning.
South Korea responded swiftly with Defence Minister Kim Kwan-Jin promising military retaliation in the event of any attack.
"If that happens, we will strike back at the origin of fire," Kim was quoted as saying by Yonhap news agency.
"We are fully prepared," Kim said, adding that troops had been put on alert along the western section of the border.
The group planning Monday's leafleting, Fighters for a Free North Korea, said they had no intention of calling off the event.
"We'll never buckle to threats from North Korea," the head of the organisation, Park Sang-Hak, told AFP.
During his visit to Yeonpyeong island on Wednesday, Lee had told troops stationed there that they should "fight to the death" to protect the border and "retaliate strongly" to any North Korea provocation.
The North shelled Yeonpyeong in the Yellow Sea on November 23, 2010, leaving two South Korean soldiers and two civilians dead. The South retaliated with its own artillery bombardment on two targets in the North.
There have been widespread concerns in the South that Pyongyang may try to instigate a military clash that would temporarily destabilise the Korean peninsula in the run up to South Korea's presidential election in December.
Some South Korean analysts played down the warning from the North, saying its main aim was psychological.
"I think this is a bluff. I don't think they mean to actually target and shell the area," Kim Yong-Hyun, a professor at Dongguk University, told AFP.
"It could be an indirect reaction to what President Lee said yesterday and the North is also seeking to drive wedges between conservatives and liberals ahead of the presidential poll," Kim said.
On Wednesday, South Korea had announced an annual, large-scale military exercise aimed at countering threats from North Korea.
The week-long Hoguk exercise beginning October 25 will involve 240,000 army, navy, air force and marine corps personnel, with 500 US soldiers also taking part.
Some 28,500 US military personnel are stationed in the South -- a legacy of the Korean War that ended with a ceasefire but not a peace treaty, leaving the two Koreas still technically at war.
