North Korea's military said earlier this week that it would "completely cut off roads and railways" linked to South Korea as relations between the two neighbours continue to worsen.
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North Korea has accused South Korea of flying drones to its capital to drop anti-regime propaganda leaflets and threatened to respond with force if such flights occur again.
The Foreign Ministry in Pyongyang said in a statement that South Korean drones were detected over the capital on three nights this week.
The ministry accused the South of violating North Korea's "sacred" sovereignty and threatening its security and described the alleged flights as a "dangerous provocation" that could escalate to an armed conflict and even war.
It said North Korean forces will prepare "all means of attack" capable of destroying the southern side of the border and the South Korean military 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 issued a denial of the allegations.
Asked about the North's claims during a parliamentary hearing, South Korean Defence Minister Kim Yong-hyun told lawmakers, "We have not done that."
It wasn't immediately clear whether Kim was referring to South Korean military drones or drones possibly operated by South Korean civilians.
The South's Joint Chiefs of Staff later said in a statement that it couldn’t 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.
North Korea is extremely sensitive to any outside criticism of the authoritarian government of leader Kim Jong Un and his family’s dynastic rule over the country.
Since May, North Korea has sent thousands of balloons carrying paper waste, plastic and other refuse to drop on the South, in what it described as retaliation against South Korean civilian activists who flew balloons with anti-North Korean propaganda leaflets across the border.
South Korea's joint chiefs said in a separate statement that the North was again flying balloons toward the South on Friday evening and warned civilians to beware of objects falling from the sky.
The South's military responded to the North's balloon campaign by using border loudspeakers to broadcast propaganda and K-pop to North Korea.
The psychological warfare campaigns have further escalated tensions created by Kim's weapons tests and verbal threats of nuclear conflict against Washington and Seoul.
The allies have responded by strengthening their combined military exercises and expanding three-way cooperation with Japan, while also upgrading nuclear deterrence plans built around US strategic assets.
On Wednesday, North Korea said it will permanently block its border with South Korea to counter what it called "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.
Experts say Kim's nuclear push is aimed at eventually forcing the United States to accept North Korea as a nuclear power and allowing the North to negotiate security and economic concessions from a position of strength.