South Korea's top office has strongly denounced the killing of the official, calling it an "inhumane act."
President Moon Jae-in demanded Thursday that North Korea must provide a response and take appropriate action against the perpetrators.
Kim Min-ji reports.
President Moon Jae-in has called North Korea's killing of a South Korean national "shocking" saying that it cannot be tolerated for any reason.
His remarks come after he was briefed Thursday on the results of the National Security Council meeting which convened to discuss response measures.
"President Moon said that North Korea must offer an explanation and take responsible measures. He also ordered the military to further strengthen its security posture and ensure it is fully prepared to protect the lives and safety of the people."
The NSC earlier issued a statement condemning the North calling the killing "inhumane" and an act that goes against international regulations.
It said that Pyeongyang must apologize and reveal the entire truth.
"North Korea must apologize and take measures to prevent this from happening again. The South Korean government will respond resolutely to any North Korean act that threatens people's lives and security."
A Blue House official told reporters that an emergency meeting was held at the top office in the early hours of Wednesday to analyze intelligence.
Hours later, President Moon was briefed on the outcome and ordered officials to confirm the details with the North Korean side and let the public know exactly what happened.
There's been no response from Pyeongyang as of yet.
When asked whether South Korea sees any involvement by North Korean leader Kim Jong-un, the Blue House said that the intelligence needs to be analyzed further.
The timing of the man's killing is notable as President Moon gave his pre-recorded UN address this week calling for a declaration to end the Korean War.
The latest development is a blow to inter-Korean relations, and puts the brakes on efforts to revive the peace process.
But the top office says that the incident is not a violation of the inter-Korean military agreement although it does go against the spirit of the deal which aims to ease tensions and build trust.
For now, the top office is expected to keep close tabs on the situation, while waiting for a response from North Korea. The Blue House said that it will let people know if there are any further developments. Kim Min-ji, Arirang News.