South Korea's defence ministry said Friday it was convinced that three crashed drones recovered in recent weeks were of North Korean origin.

Wreckage of the rudimentary drones was found in three different places, including Baengnyeong island near the rivals' disputed sea border.

"By inspecting the three aircraft and their loaded devices, we've secured various pieces of circumstantial evidence that all certainly point to North Korea", the defence ministry said in a press statement.

All three unmanned aerial vehicles -- about one metre (three feet) long, two metres wide and painted pale blue -- were equipped with cameras.

Their memory chips contained pictures of border areas and the capital Seoul, including the presidential palace, although they were of low quality, the ministry said.

Their constituent parts were from various countries, including South Korea, the United States, Japan and China, and were all widely available.

However, any serial numbers had been "intentionally removed," the ministry said, adding that US forensic experts would help with further analysis of the drones.