SEOUL (AFP) - Thousands of North Korean propaganda leaflets fell on Monday on a frontline South Korean island, warning of an attack on soldiers stationed there, a report said.

A South Korean marine unit based on Baengnyeong Island in the Yellow Sea had collected the leaflets which were carried across the border, Yonhap news agency said.

The South's military declined to confirm the report.

The leaflets carried threats of bombings, describing the marines as the first target to be wiped out.

The North also warned in the leaflets that its troops were always ready to strike the island with their "unprecedented" fire power and turn it into "a large graveyard", Yonhap said.

The warning came days after North Korea executed Jang Song Thaek, long seen as the country's unofficial number two and leader Kim Jong Un's political regent, for a host of charges including plotting to overthrow his nephew and corruption.

The execution was the biggest political upheaval since Mr Kim took power after the death of his father two years ago.

South Korea has been on alert for potential fallout from the purge north of the border with President Park Geun Hye warning of possible "reckless provocations" by Pyongyang.

The two Koreas' Yellow Sea border was the scene of several deadly naval clashes. In 2010, a South Korean naval vessel, the Cheonan, was sunk near Baengnyeong with the loss of 46 lives.

South Korean residents on frontline islands have lived in the constant fear of possible attacks since the North's shelling of an island killed four people in late 2010.

In 2004 the two sides agreed to stop all official-level cross-border propaganda. But the South's military resumed floating anti-Pyongyang leaflets in late 2010 as military tensions soared. The North resumed its own propaganda exercise in 2012.

South Korean activists including defectors from the North regularly launch anti-Pyongyang leaflets across the border by balloon, despite the North's threats to shell them.