South Korea's Defence Ministry said Thursday that it had "smoking gun" proof that three crashed drones recovered in recent months had all been flown from North Korea.

Ministry spokesman Kim Min-Seok said a joint investigation with US experts of recovered data from the unmanned aerial vehicles (UAVs) showed they had been pre-programmed to fly over South Korean military installations and then return to the North.

"By analysing the data, the joint investigation team has secured the smoking gun -- clear, scientific evidence that all three UAVs originated from North Korea," Kim said.

"This is a clear military provocation," he added.

The drones were recovered in three different locations in the South near the inter-Korean land-and-sea border between March 24 and April 6.

One crashed due to an engine problem, while the other two ran out of fuel.

"All three had been programmed to fly over our military facilities, Kim said, describing the drone incursions as "a new type" of military threat that required a "stern" response.

In a separate briefing with foreign journalists, Vice Defence Minister Baek Seung-Joo said the drones were unsophisticated UAVs with no live ground-control system and rudimentary programming systems.

"However, there is always the possibility that the North might use them for attacks after arming them with high explosives, considering its irrational and reckless tendency for provocative acts," Baek said.

North Korea has flatly denied any connection with the drones, and accused Seoul of fabricating a link in order to smear Pyongyang.

All three UAVs were equipped with cameras and had taken pictures of border areas and the capital Seoul, including the presidential palace.

North Korea had displayed a set of what looked like very basic drones during a huge military parade held in Pyongyang last July to mark the 60th anniversary of the end of the Korean War.

And in March last year, state media reported leader Kim Jong-Un overseeing a military drill using "super-precision drone planes."

Video footage of the exercise broadcast on state television showed what resembled air force target drones being flown into a mountainside and exploding.

The South Korean defence ministry said the North had an estimated 300 drones of various types, and said it planned to acquire low-altitude surveillance radar to counter their threat.

There is evidence to suggest the North Korean models were based on a Chinese-made UAV and the ministry said Beijing had been asked to check into a possible link between the Chinese manufacturer and Pyongyang.

The data recovered from the crashed vehicles showed they had all been flown from different locations, between five and 30 kilometres (three and 18 miles) inside the North Korean side of the border.