Image copyright EPA Image caption The demilitarized zone is one of the most heavily fortified borders in the world

South Korea has warned of a "pitiless penalty" for North Korea, after blaming it for a landmine blast in the demilitarized zone (DMZ) last week.

One South Korean soldier had both legs amputated after the blast, while another lost one leg.

South Korea initially said there was no indication of North Korean involvement.

But a defence ministry official said on Monday that Seoul was "certain they were North Korean landmines planted with the intention to kill".

Officials said North Koreans had sneaked across the heavily protected border to plant three mines close to a South Korean border post.

The Joint Chiefs of Staff said in a statement it was "a clear violation of the armistice agreement and the non-aggression agreements between the South and the North".

"As previously warned on many occasions, our military will make North Korea pay the equally pitiless penalty for their provocations," it said.

The 4km (2.5 mile)-wide DMZ is one of the world's most protected borders, separating the two Koreas which remain technically at war.