North and South Korea exchanged land fire today, Yonhap news agency reports, after a group of South Korean activists launched propaganda balloons across the border. South Korean military officials tell Yonhap that North Korea targeted the balloons and that the South Korean army returned fire after some shots landed on their side of the border. The two sides exchanged machine gun fire, though there as yet no reported casualties.

The incident comes as North Korea's ruling Workers' Party celebrates its 69th anniversary, and amid widespread speculation about the health and leadership of leader Kim Jong-un, who has not been seen in public in more than a month. He was not in attendance at today's celebration, and North Korean state-run media have said only that Kim is suffering from an unspecified physical condition.

The ten ballons were launched from the city of Paju and contained 200,000 leaflets condemning Kim and his regime. They also contained 400 DVDs and 300 books about life in South Korea, as well as $1,000 in cash. A group of North Korean defectors used the same tactic in January. North Korea is among the most isolated regimes in the world, and both Kim and his father, Kim Jong-il, have maintained tight control over communications.

Pyongyang and Seoul have exchanged maritime fire on several occasions, but as the BBC notes, clashes on land are less common.