Seoul, South Korea - North Korea said Friday that the South had sent unmanned drones carrying propaganda leaflets to Pyongyang three times, a claim that Seoul has denied.

Relations between the two Koreas are at one of their lowest points in years, with Kim Jong Un declaring South Korea his country's "principal enemy" earlier this year.

Citing the North's foreign ministry, the official Korean Central News Agency reported Friday that the South had sent drones into the Pyongyang airspace on October 3, and then again on Wednesday and Thursday this week.

The drones were dropping anti-regime propaganda, KCNA reported, adding that the leaflets were "filled with heinous slander."

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The North issued a "final ultimatum" warning South Korea to immediately cease the so-called provocations.

Flying drones into Pyongyang's airspace "could be considered a military attack," the foreign ministry said according to KCNA, adding it is "an intolerable and unforgivable grave provocation."

"This incursion into our airspace is a serious crime that violates our sovereignty and is a clear justification for exercising our right to self-defense. South Korea has chosen its own destruction. It is hastening its downfall."

Photographs of the alleged propaganda leaflets were shared by state media, alongside a blurry picture with a so-called "unmanned drone" marked out.

"All of our attack forces... are ready to act at any moment," the foreign ministry added.

South Korea's Defense Minister Kim Yong-hyun said South Korea had not sent any drones into the North.

"We have not done that," he told lawmakers when asked about North Korea's claim during a parliamentary audit late Friday.

An official from Seoul's Joint Chiefs of Staff also told reporters that the military "has never sent an unmanned drone to North Korea" and they are confirming if it was done by "civic groups," according to South Korea's Yonhap News Agency.

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Despite official efforts to prevent them, South Korean activists have for years flown balloons containing propaganda leaflets and US dollars over the border, a tactic which infuriates Pyongyang.

In response, North Korea has sent over 6,000 trash-filled balloons south since May, saying they are retaliation.

Yonhap reported that Pyongyang was sending a fresh round of trash-carrying balloons late Friday.

The Friday KCNA statement comes days after the North's army said it would "permanently shut off and block the southern border" with South Korea.