South Korea's defense minister ordered tighter vigilance against new threats of small unmanned aerial vehicles on Monday as a recent discovery of suspected North Korean drones has sparked concern over the nation's air defense system.

Kim Kwan-jin issued the order at a meeting of top military commanders convened to discuss anti-drone measures after South Korea collected three drones in less than a month near the western and eastern front and on a border island.

One was found in late March in Paju, just south of the demilitarized zone separating the two Koreas, and the other was retrieved from the front-line island of Baengnyeong during the North's live-fire drills near the western maritime border last week.

Most recently, the military on Sunday revealed a third drone similar to the other two drones, which a local resident found in a mountain on the east coast in early October.

"If (North Korea) developed the small unmanned aerial aircraft for reconnaissance purposes to enhance its relatively weak surveillance capability, it is expected to develop drones for secret infiltration and terrorism purposes in the future," Kim said during the video conference held at the Joint Chiefs of Staff headquarters in Seoul.

"We should strengthen our military readiness to be able to monitor, detect, identify and strike (the drones) with existing military assets along the border."

Rudimentary drones have existed for almost as long as aircraft themselves for reconnaissance and surveillance missions have, but the drone incursions have raised concerns over more sophisticated, armed UAVs that can launch attacks on military assets and major facilities.

Seoul officials have discussed anti-UAV measures in the past years, but so far, defenses against drones mostly consist of signal jamming, radar detection and conventional counterattack used against other types of aircraft.

In light of the drone incursions, Seoul's defense ministry said it is considering purchasing advanced low-altitude surveillance radars and anti-aircraft guns to better guard against infiltration by small aircraft.

With an in-depth analysis current underway, the three sky blue drones equipped with cameras, which crashed either due to engine failure or lack of fuels, showed Pyongyang's UAV technology to be at a rudimentary level.

In light of the drone discovery made by reports from local residents, military units across the country began searching for other drones that may have crashed in South Korea.

During the meeting, the defense chief also pledged to cooperate with related government agencies and civilians to better spot the small aircraft, which are hard to detect with the existing radars.

As part of the measures, the military is considering giving a monetary reward to any informant who reports the existence of a drone, according to officials.

The latest details from the investigation showed that the drones found in Paju and Samcheok may have extended their flight range by having had their glow engines modified to run on a more effective gasoline fuel, said a military official with knowledge of the ongoing investigation. The glow engine is a small internal combustion engine typically used in model aircraft.

While the triangle-shaped drones with an average speed of 100-120 km per hour are believed to have flown as far as 200 km, the Samcheok drone was discovered 130 km south of the military demarcation line. It is a strong indication that the engine may have used more cost effective gasoline fuel as it is designed to fly a round trip covering a distance of over 260 km, the official said.

"The joint investigation team has assembled the drones to look into engines to figure out whether they were modified to gasoline engines to extend their flight range," the official said. "An in-depth analysis of the engine structure would provide (knowledge about the North's) technology to make drones that can fly according to prearranged coordinates unaffected by wind."

If the North successfully modified the glow engine to run on gasoline mix, it could conduct spy missions farther south, he added.

Experts say North Korea is believed to have deployed about 100 attack drones it unveiled last March during military drills, which was lauded by leader Kim Jong-un.

The range of the drones is estimated at up to 800 kilometers, sufficient to strike major South Korean and U.S. military targets in South Korea.

The North on Saturday derided the discovery of the drones that flew undetected over key areas in Seoul that tarnished the image of the South Korean forces, but made no mention of its involvement.

If the ongoing probe confirms that the drones were sent by the North, the spokesman for Seoul's defense ministry said it would constitute a violation of the Armistice Agreement of the 1950-53 Korean War and international regulations, and vowed to take "relevant actions" without elaborating on the details of potential measures.

The Joint Chiefs of Staff said Sunday it will take necessary "military and legal actions" over the North's violation of its airspace if the drones are found to have come from the country. (Yonhap)