A South Korean military inquiry into a drone found on a border island has concluded that North Korea flew the unmanned aircraft to conduct reconnaissance missions, a South Korean media report said on Wednesday.

The discovery of the surveillance aircraft came less than an hour after a three-hour artillery barrage between South and North Korea in each side's territorial waters near a disputed maritime border on Monday.

South Korea's defence ministry declined to comment on the report as its probe was under way.

North Korea fired more than 100 artillery rounds into South Korean waters as part of a drill on Monday, prompting the South to fire back. The exercise appeared to be more sabre-rattling by Pyongyang rather than the start of a military standoff.

Yonhap News Agency reported on Wednesday that the drone's flight route appeared to be from the North, citing unidentified South Korean government officials.

2nd drone investigated

South Korean military officials said they were also investigating a similar drone found in a border city late last month.

A defence ministry official told a briefing that North Korean-style writing was inscribed on the battery of that drone and its flight route was set up to return north. But it had not yet concluded that the drone was sent by North Korea as investigations proceeded.

Images of Monday's crashed drone showed the wreckage of a small, light-blue aircraft bearing similar paintwork and markings to North Korean drones displayed in a Pyongyang parade last year.

Those drones were larger aircraft modified to crash into pre-determined targets, but are not believed to be capable of air strikes or long-range surveillance flights.

North Korea's state media said last year its leader Kim Jong-un had supervised a drill of "super-precision" drone attacks on a simulated South Korean target.

Experts said the crashed drone was an old, poorly-designed model. Although the North has one of the world's largest standing armies, much of its equipment consists of antiquated Soviet-era designs.

"It is like a toy. But for surveillance purposes, it doesn't have to be a high-tech, top-notch military product like Predators or Global Hawk drones," said Kim Hyoung-joong, a cyber defence professor at Korea University in Seoul.

"This type of toy-like equipment can find a blind spot."