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YEONCHEON VILLAGE, South Korea — There are a lot of reasons why the world's most fortified border is scary, but what has really rattled North Korean leader Kim Jong Un are 11 loudspeaker installations hidden among the trees just south of the boundary.

The nuclear-armed North has condemned South Korean propaganda being blasted from the speakers as equivalent to a declaration of war.

It was this that triggered Thursday's exchange of fire across the demilitarized zone, or DMZ, and Kim's latest ultimatum — stop the broadcasts by 5 p.m. Saturday (4:30 a.m. ET) or face military action.

He has placed his army on what the North calls a "quasi state of war."

What makes this incident more worrying is that this time the North followed through with a threat, which usually doesn't happen. And the South hit back robustly, something that is also rare.

There don't seem to have been any casualties or damage on either side, suggesting that for all the sound and fury the targets were most likely rocks or hills designed to cause the least amount damage.

Still, South Korean authorities have evacuated several villages close to where the shelling happened in Yeoncheon District.

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Near one shelter visited by NBC News, Korean Red Cross workers prepared food while around 20 villagers — mostly elderly women and children — were in a concrete bunker reached by steep stairs and a heavy blast-proof door.

"They are upset more by the quality of the speakers than the content itself"

"I thought at first it was thunder," 10-year-old Yoo Jin said referring to Thursday's artillery exchange. "But then it went on and on, and I knew that wasn't thunder."

Jang Dongsoon, who had lived in the area for 33 years, said she has grown used to threats from the North — and the occasional bang from the DMZ.

"North Korea makes threats all the time, but usually doesn't do anything. So I am not that nervous," she said.

The evacuees sat around watching television — a rolling bulletin about the border crisis. Youngsters texted and played video games on their telephones. If there were fears about what the weekend might bring they were not showing it.

Still, there has been a definite ratcheting-up of tensions in the last two weeks.

A South Korean loudspeaker meant to blast anti-Pyongyang propaganda sits just south of the border with North Korea on August 11. South Korea Defense Ministry / EPA

Blasting propaganda across the border used to be a routine weapon of both Koreas. But the broadcasts stopped in 2004.

Seoul resumed the broadcasts 11 days ago after two South Korean soldiers were badly injured by a freshly-laid mine in the DMZ. The incident caused anger and outrage in the South. The North later followed suit.

The latest broadcasts focus on the superiority of life in South Korea — as well as South Korean soap operas. North Korean soldiers station near the border are also regaled with K-Pop, accounts of how defectors are embraced by the South and weather reports.

Cha Duhyeogn, a North Korea expert at the Korea Institute for National Unification said that Kim's regime is also upset because the South's loudspeakers are much more high tech and generally superior to their own hissing efforts.

"They are upset more by the quality of the speakers than the content itself," Cha said.

On Thursday, North Korea fired two shells into the South, apparently in protest at the broadcasts. The South hit back with a barrage of 29 shells of its own.

Officials in Seoul say the South will continue the broadcasts and will respond forcefully to any provocation over the weekend.