SEOUL: North Korea threatened on Friday to carry out a “merciless military strike” on South Korea next week, prompting a swift vow of retaliation from Seoul in a serious escalation of cross-border tensions.

The Korean People’s Army (KPA) vowed to initiate the strike if a group of North Korean defectors now living in the South went ahead with plans to scatter propaganda leaflets next 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 border that was shelled by Pyongyang 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 on the official Korean Central News Agency.

Residents in and around the area where the activists plan to launch their balloons should “evacuate in anticipation of possible damage,” the statement said.

“The surrounding area will become targets of direct firing of the KPA,” it said, adding: “The KPA never makes empty talk.”

A group of North Korean defectors plan to carry out the leafleting exercise on October 22 at 11:30 a.m. (10:30 a.m. in Manila) at the border near the town of Paju, around 60 kilometers north of Seoul.

Such exercises 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 Defense 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.

The group planning Monday’s leafleting, Fighters for a Free North Korea, said that they had no intention of calling off the event.

“We’ll never buckle to threats from North Korea,” the head of the group, Park Sang-Hak, said. “We will push through with it and stick to the plan down to the minute.”

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” in the event of any North Korea provocation.

There have been widespread concerns in the South that Pyongyang may try to instigate a military clash that would temporarily destabilize the Korean peninsula in the run up to South Korea’s presidential election in December.

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.