Pyongyang on Tuesday blew up deeply symbolic roads and railways connecting the two Koreas after warning any further drone flights would be considered a declaration of war, and ordered soldiers on the border to prepare to fire.

Seoul initially denied sending drones, but Pyongyang claims it has "clear evidence" of official involvement in the campaign, which purportedly features anti-regime propaganda leaflets scattered over the North's capital.

"Millions of young people have turned out in the nationwide struggle to wipe out the ROK scum who committed a serious provocation of violating the sovereignty of the DPRK through a drone infiltration," the official Korean Central News Agency said, referring to both countries by their official acronyms.

It said more than 1.4 million youth league officials and youth and students across the country volunteered to join or rejoin the Korean People's Army on October 14 and 15.

North Korea already has lengthy periods of mandatory military service for all men, and has previously made claims of patriotic waves of enlistments at times of high tensions with Seoul or Washington.

While it remains unclear who is behind the drone flights, South Korean activists have long flown balloons carrying anti-Pyongyang regime leaflets over the border, a tactic that infuriates the North and which has responded by bombarding the South with trash-carrying balloons.

South Korean authorities in areas near the border with the nuclear-armed North are moving to prevent activists from launching balloons.

To protect its citizens, the provincial government of Gyeonggi will designate Yeoncheon, Gimpo and Paju "as special 'danger' zones where anyone trying to send leaflets to the North may be subject to criminal investigation", an officer from the Gyeonggi provincial goverment told AFP.

"The Gyeonggi Province considers the act of distributing anti-North leaflets to be a crisis-causing dangerous act that could cause a military conflict," the Gyeonggi government said in a separate statement.

- 'Unsettling broadcasts' -

South Korean activists -- many are former North Koreans who defected -- have also sent materials such as USB drives containing K-pop tracks and K-dramas. The isolated North is extremely sensitive about its people gaining access to the South's popular culture.

In response to the North's trash balloons, Seoul suspended a tension-reducing military deal and resumed loudspeaker broadcasts along the border, which feature K-pop and international news.

Since late July, Pyongyang has broadcast eerie sounds along the border -- some resembling the cries of wild animals -- in apparent retaliation.

"The anxiety and suffering of residents in border areas is becoming increasingly severe," the Gyeonggi province said in the statement.

"Many are reporting that they can't sleep without medication due to the constant influx of garbage balloons and unsettling broadcasts from the North."

In 2020 South Korea's parliament passed a law banning the sending of anti-Kim leaflets across the border as then-president Moon Jae-in sought engagement with the North.

But the Constitutional Court struck down the law last year, calling it an undue limitation on free speech.