SEOUL - The Defense Ministry of the Republic of Korea (ROK)?said Thursday that three drones discovered around the inter-Korean border were sent by the Democratic People's Republic of Korea (DPRK).

Defense Ministry spokesman Kim Min-seok told a press briefing that the US-ROK joint investigation team conducted a scientific probe into three DPRK drones after announcing an interim probe result on April 11.

Kim said three drones were pre-arranged to take off from and return to the DPRK based on investigation into the built-in memory chips. The unmanned aerial vehicles (UAVs) flew in accordance with pre-arranged coordinates via the global positioning system (GPS).

Kim called flight paths collected from the pre-arranged coordinates as a "smoking gun" to prove the drones came from the DPRK, saying the pilotless airplanes flew over major ROK military installations.

The first DPRK drone was found on March 24 in Paju, the northwestern ROK city close to the land border. The second one was discovered in the Baengnyeong Island just south of the disputed western sea boundary on March 31 when the two Koreas exchanged artillery fires.

One more UAV, flown by the DPRK possibly last year, was spotted on April 6 in Samcheok, around 290 km east of Seoul and some 130 km south of the military demarcation line between the two Koreas.

The drones were equipped with digital cameras, capturing images of military installations near the Demilitarized zone and the Blue House in Seoul where President Park Geun-hye's residence and office are located.

The ROK military said earlier that the drones were not sophisticated enough to send images in real-time and the quality of images was no better than those from commercial satellites. The UAVs were believed to carry just 2-3 kilograms of TNT for an attack purpose, which will not cause a serious damage.

Concerns remained that the DPRK drones can be modified to carry chemical and biological explosives and crash into somewhere in the capital area of the South. Defense Minister Kim Kwan-jin said in early April that the drones will possibly advance into a "suicide bomber" though they looked rudimentary yet for the reconnaissance purpose.

President Park said insecurities were growing among citizens in ROK, especially people living near the border area, noting that though DPRK drones spied in all directions, the military was completely in the dark about it.

The military planned to purchase advanced low-altitude radars to detect and strike the small drones. The ROK's army was operating low-altitude radars called TPS-830k, but they were unable to spot the small UAVs.

The spokesman said that this DPRK act is a clear military provocation and a violation of armistice agreement that banned mutual aggression, noting the military will take a stern response to such provocation and issue a warning against the DPRK through the UN command.

The ROK military will view the small DPRK drones as a new threat and push to strengthen its air defense capability, Kim said, noting it will build up a comprehensive defense system that can detect and strike the drones.

The DPRK has denied having sent the drones to the South, claiming it was fabricated by ROK. The DPRK's Committee for the Peaceful Reunification of Korea said after the interim probe result announcement that ROK was fabricating another "Cheonan incident," calling it Seoul's smear campaign against Pyongyang.

The Cheonan incident referred to the sinking of the ROK's navy corvette Cheonan, which killed 46 soldiers. The warship sank in waters near the western maritime border in what a Seoul-led multinational investigation team claimed was a torpedo attack by Pyongyang. The DPRK has repeatedly denied its involvement in the incident.