Seoul on Friday accused the Democratic People's Republic of Korea of a "blatant lie" in claiming it had not fired shells near a Republic of Korea warship, and said Pyongyang had threatened to bombard its military vessels.

Earlier on Friday, the DPRK military rejected as "sheer fabrication" Seoul's assertion that two shells had been fired in the vicinity of a navy vessel on patrol near the tense sea border on Thursday.

Seoul's Defense Ministry said the shells fell about 150 meters from an ROK navy corvette near the disputed sea border in the Yellow Sea.

The ship was not damaged and responded by firing five rounds into the water near a DPRK military vessel.

"The verified fact is that the puppet navy vessel, which intruded deeply into our waters under the pretense of controlling Chinese fishing boats, fired recklessly and lied that we had fired first. This is a sheer fabrication," the DPRK military's Western Front Command said in a statement.

"All the troops under the Western Front Command are well prepared to crush ruthlessly the aggravating provocative acts by the puppet military gangsters in the name of all the people," it said.

It then vowed to turn the tense sea border area into "tombs" for the ROK military.

But ROK Defense Ministry spokesman Kim Min-seok stood by Seoul's charges and dismissed the DPRK's claim as a "blatant lie".

He said that the DPRK's "far-fetched claims are nothing but a blatant lie ... and are subject to ridicule by the international community".

A military Joint Chiefs of Staff spokesman from the ROK said on Friday that the DPRK continuously sent messages through an international radio channel threatening attacks on the ROK military vessels operating near the sea border.

"Recently, it has been threatening to bombard our ships unless they pull back," he said.

ROK President Park Geun-hye expressed "very strong regret" on Friday that the DPRK committed this "provocation" at a time when the ROK is still grieving over last month's ferry sinking, her spokesman Min Kyung-wook said.

The buildup to the incident started on Tuesday when a naval ship of the ROK fired warning shots to turn back three DPRK patrol boats that had crossed the disputed sea boundary.

The DPRK then threatened on Wednesday to launch an attack on ROK warships without warning at the slightest hint of any provocative act.

Republic of Korea President Park Geun-hye (center) presides over a security meeting at the Presidential Blue House in Seoul on Friday. The Presidential Blue House via Reuters