South Korea's Joint Chiefs of Staff said Wednesday they found strongly worded North Korean leaflets in the Han River, the first distribution of such propaganda materials via a waterway.

On July 22, the Korean military collected scores of tightly air-filled vinyl bags carrying North Korean leaflets which contained threats of the North's possible attacks on the South using the intermediate-range Musudan ballistic missile, a spokesman for Seoul's JCS said in a press briefing.

"North Korea intentionally floated the leaflets toward the mouth of the Han River and were picked up by guard troops near Gimpo," the spokesman said, citing an analysis by relevant government officials. The river flows through Seoul, South Korea's capital city, although sources said it is not clear where the leaflets originated from.

The leaflets also carried a message celebrating the North's "victory" against the U.N. Forces during the Korean War (1950-53).

On Wednesday, two Koreas separately marked the 63rd anniversary of the Korean War Armistice Agreement that resulted in the conflict ending in a ceasefire agreement and a stalemate. The North, however, has been insisting it won the war and that the armistice was only reached because the U.N. forces sued for peace.

The JCS said the military will strengthen its monitoring and keep soldiers on high alert to detect additional North Korean propaganda activities along the mouth of the Han River and other vulnerable areas.

The leaflet distribution is the latest provocation by the North after it test-fired three ballistic missiles on July 19 in protest against the South's decision to deploy an advanced U.S. anti-missile system by 2017.

Seoul said the anti-missile system is needed to counter growing threats from the North.  (Yonhap)