South Korean military said Sunday it had found an unidentified "drone" suspected to have flown from North Korea, following the discovery of two similar objects near the border in recent weeks.

The South earlier found two unsophisticated unmanned aerial vehicles -- about one metre long, two metres wide and painted pale blue -- that had crashed near the border with the North.

The two camera-equipped vehicles took hundreds of pictures -- albeit of low quality -- of border areas and the capital Seoul, including the presidential palace, according to the South's defence ministry.

The military found Sunday another drone of similar design and size in the eastern county of Jeongseon, about 130 kilometres (80.7 miles) south of the heavily fortified border, Seoul's defence ministry spokesman said.

"We ... will conduct investigations on the object on the suspicion that the North may be behind it," Kim told reporters.

Seoul's military saw the drones as "substantial threats" and would come up with ways to detect them, Kim said without elaborating further.

The first drone was discovered in the border city of Paju on March 24 and the other one was found in the border island of Baengnyeong on March 31, when the two Koreas traded fire across the tense sea border.

The North dropped 100 shells across the maritime border during a live-fire drill, prompting Seoul to fire back about 300 volleys into North Korean waters.

The military said Wednesday it had evidence suggesting the vehicle found in Paju was of North Korean origin, citing unique North Korean words typed on its batteries.

North Korea displayed a set of very basic-looking drones during a huge military parade held in Pyongyang last July to celebrate 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."

The North on Saturday jeered at the discovery of drones that apparently flew freely over key areas in Seoul -- but offered no clear denial or confirmation as to its involvement.

The drone cases "occurred all of a sudden, more badly tarnishing the image of the (South's) forces which had fallen to the ground," state-run KCNA quoted the North's army spokesman as saying.