The North's Korean Central News Agency issued the threat of "unpredictable and merciless" fire, the South's Yonhap reported.
South Korean activists and defectors from the North floated 200,000 leaflets by balloon over the demilitarized zone April 15, bringing news of the Arab uprisings and calling on North Koreans to overthrow the Kim Jong Il regime. April 15 is the birthday of North Korea's founder, Kim Il Sung and is a national holiday.
Friday's statement called the leafleting "a form of psychological warfare and a clear-cut war provocation. … Direct fire at the area where leaflets are let fly will be a legitimate punishment."
South Korean activists floated 300,000 more leaflets Friday, and Lee Min-bok, organizer of the campaign, said Pyongyang's anger shows it is effective.
The North also protested the firing of machine guns near the border April 15 as an "unpardonable military provocation." The South Korean Defense Ministry said it was an accident.