North Korea has accused neighbour 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 on Friday that South Korean drones were detected in the night skies of Pyongyang on October 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.”

Asked about the North Korean claims during a parliamentary hearing, South Korean defence minister Kim Yong-hyun told lawmakers: “We have not done that.”

He said he was still trying to assess the situation and did not elaborate further.

It was not immediately clear whether Mr Kim was referring to South Korean military drones, or also drones possibly operated by South Korean civilians.

The South’s joint chiefs of staff later said in a statement that it could not confirm whether the North’s claims were true, without elaborating why. The joint chiefs warned the North to “exercise restraint and not act recklessly”.

“If the safety of our citizens is threatened in any way, our military will respond with stern and thorough retaliation,” it said.

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.

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 US 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 US strategic assets and its rivals’ rhetoric.