South Korea lashed out at North Korea on Friday for denying its surprise artillery firing in the western sea border the previous day, calling it a "blatant lie."

Seoul's defense ministry said Thursday the North fired two shells into waters near a South Korean warship on patrol south of the maritime border in the Yellow Sea, prompting the patrol boat to return fire. No casualties or damage were reported.

Earlier Friday, the North flatly denied it fired the artillery, saying the South Korean warship crossed its border to launch an attack and that Seoul was trying to fabricate the incident.

"North Korea's such far-fetched claims are nothing but a blatant lie to shun responsibility and are subject to ridicule by the international community," defense ministry spokesman Kim Min-seok said at a regular briefing.

"We already sent a stern warning against the North yesterday and made it clear that we will respond squarely in case of its continued provocations."

The North Korean military has continued to threaten to attack with artillery in recent days against the South's warships on patrol near the NLL unless they withdraw, according to military officers.

Kim also said the South's military had "pieces of physical evidence" that the North made such a surprise attack, while noting that it is "not 100 percent clear yet" if the communist country fired the shells with coastal artillery.

"North Korea fired only a couple of shells, which makes it difficult to know exactly where they came from ... But what we've learned so far indicates that the attack did not directly target our vessel," the spokesman added.

Thursday's attack came after the communist country has ratcheted up tensions on the Korean Peninsula, making good on its military threats.

Earlier this week, the South Korean Navy fired warning shots to disperse military vessels from the North that crossed the sea border into the southern side. In return, the unruly nation threatened to target South Korean warships.

Thursday's artillery firing was the latest in a series of North Korean provocations. In November 2010, the North shelled Yeonpyeong Island south of the western sea border, killing four people, including two civilians.

Eight months earlier, North Korea torpedoed the South Korean warship Cheonan in the Yellow Sea, killing 46 sailors. Pyongyang has denied its involvement in the incident. (Yonhap)