North Korea has accused rival South Korea of flying drones to its capital to drop anti-North Korean propaganda leaflets and threatened to respond with force if such flights occur again.

North Korea's Foreign Ministry said in a statement Friday that South Korean drones were detected in the night skies of Pyongyang on Oct. 3 and Wednesday and Thursday this week.

The ministry accused the South of violating North Korea's "sacred" sovereignty and threatening its security, and said its forces will prepare "all means of attack" and respond without warning if South Korean drones are detected in its territory again.

"The safety lock on our trigger has now been released," the ministry said. "We will be prepared for everything and will be watching. The criminals should no longer gamble with the lives of their citizens."

South Korea's military said it had not flown any drones into North Korea, Yonhap news agency reported.

North says it plans to permanently block border

Tensions between the rival Koreas have escalated in recent months as North Korean leader Kim Jong-un has ramped up weapons tests and threats and South Korea has responded by strengthening its joint military exercises with the United States.

Since May, North Korea has also sent thousands of balloons carrying paper waste, plastic and other trash to drop on the South, in a bizarre psychological warfare campaign that worsened the animosity between the nations.

WATCH l Explaining the trash controversy between the Koreas:

Why North Korea’s dumping garbage on South Korea

North Korea sent hundreds of balloons carrying cigarette butts, batteries and manure over South Korea. Andrew Chang explains why, and how this plays into a bigger, much more serious picture.

On Wednesday, North Korea said it will permanently block its border with South Korea and build front-line defence structures to cope with "confrontational hysteria" by South Korean and U.S. forces.

North Korea's military said in a statement on state media that it will "completely cut off roads and railways" linked to South Korea and "fortify the relevant areas of our side with strong defence structures."

North Korea called its steps a "self-defensive measure for inhibiting war and defending the security" of the country and accused its rivals of "getting ever more reckless in their confrontational hysteria."

North Korea cited what it called various military exercises in South Korea, the deployment of U.S. strategic assets and its rivals' harsh rhetoric.