According to the reports, 10 packages loaded with North Korean currency and leaflets criticizing the North's leader, Kim Jong-il, were launched from a bridge near the border using balloons made from polyethylene plastic used for covering hothouses.
Pyongyang considers the act a breach of inter-Korean agreements on halting all "attacks" on the government. Seoul ignores such acts as they are not organized by the government, but it has said the unsanctioned sending of foreign currency over the border is illegal.
The South Korean government stopped official propaganda against the North in 2004, in line with a bilateral agreement. However, while discouraging activists from launching leaflets across the border, the government has refused to take action against them, citing freedom of speech.
Relations between Seoul and the communist North have deteriorated since South Korean President Lee Myung-bak took office in February 2008, rolling back the so-called Sunshine Policy of the previous 10 years.
There are intense international concerns over North Korea's announced April launch of what it says is a satellite, but what is widely suspected in the West as a cover-up for testing of ballistic missiles capable of carrying nuclear warheads.
Tensions on the peninsula have also been raised by South Korean-U.S. military exercises due to end on Friday. North Korea earlier said it could not guarantee the safety of civilian aircraft near or over its airspace, because of the exercises.
North Korea said recently it would scrap all political and military agreements with South Korea, including a non-aggression pact, over its neighbor's "hostile intent." The two countries are still technically at war as their 1950-53 conflict ended in a truce rather than a peace treaty.
