South Korea announced on Thursday that it had reached the conclusion that three surveillance drones found near the border belonged to North Korea. A joint U.S.-South Korean probe discovered flight plans stored on the unmanned crafts that traced back to locations in North Korea.

The discovery has fueled worry that the secretive and erratic northern country might soon use armed drones against South Korea. In response, Seoul’s Defense Ministry announced that it had increased airspace surveillance along the inter-Korean border and readied surface-to-air artillery.

Two of the three drones, discovered in late March and early April, had photos on them matching their flight plans, while the data on the third could not be recovered. According to the ministry, the plans were to photograph South Korean military facilities.

According to Reuters, some of the drone components were manufactured in China, Japan, the Czech Republic and the United States.

North Korea denied any involvement.