North Korea accused South Korea's military of opening fire on Sunday towards its side of the tense land border, in what it termed a "grave armed provocation".
It said the South's military fired 90 mm recoilless guns towards a civil police post in the North on the eastern sector of the border in the early afternoon.
The firing, "seriously threatening the safety of civil policemen of the north side on routine duty", was designed "to deliberately aggravate the situation" in the border buffer zone, the official Korean Central News Agency said.
A "touch-and-go situation" prevails along the border due to such undisguised military provocation, it said.
South Korea's military denied the North's claim, Yonhap news agency reported.
"We checked whether the report is true but the command and control post said there has been no such incident," it cited a military official as saying.
Seoul's forces have been ordered on heightened alert since the mysterious sinking of a South Korean warship near the disputed sea border with the North on March 26.
South Korea has not so far accused the North of involvement in the sinking, while the North has remained silent about the incident which left 46 sailors missing and feared dead.
North Korean accusations of provocation along the heavily fortified land border are not new, and there have been occasional exchanges of fire.
A buffer strip known as the Demilitarised Zone extends for two kilometres (1.25 miles) on each side of the actual borderline.