North Korea warned Friday that it was prepared to fire on sight without warning at South Korean naval vessels it accused of violating their disputed Yellow Sea border.

The Korean People's Army (KPA) said 17 South Korean high-speed patrol boats had crossed into the North's territorial waters in the first week of May "under the pretext" of intercepting Chinese fishing boats.

Issuing what it called an "emergency special warning", the KPA said the North Korean military would henceforth "make a sighting strike without any prior warning" on any South Korean naval ship deemed to have entered its waters.

In the event of any armed response, the KPA will "successively deal strong second, third and more retaliatory strikes at them," according to a statement carried by the North's official KCNA news agency.

The South Korean defence ministry flatly denied the charges of incursions by its vessels, and accused the North of seeking to stir up tensions.

In a message to the KPA command, ministry spokesman Kim Nin-Seok said any aggression would be met with a strong, stern response "to the degree where you will bitterly repent".

The two Koreas recognise different boundaries dividing their territorial waters in the Yellow Sea.

The North argues that the demarcation recognised by the South -- the Northern Limit Line (NLL) -- is invalid because it was unilaterally drawn by US-led United Nations forces after the 1950-53 Korean War.

The North recognises what it calls the Military Demilitarization Line, which is south of the NLL.

Because the Korean conflict ended with a ceasefire rather than a treaty, the two Koreas remain technically at war.

The maritime border has always been a flashpoint and was the scene of brief but bloody naval clashes in 1999, 2002 and 2009.

In November 2010, North Korea shelled Yeonpyeong island, killing four South Koreans and briefly triggering concerns of a full-scale conflict.

Most recently, North and South Korean naval patrol boats briefly exchanged warning fire in October last year.

The situation is further complicated by the seasonal arrival of hundreds of Chinese trawlers which South Korea accuses of fishing illegally in its sovereign waters.

The South Korean coastguard and navy expends significant resources chasing and seizing the Chinese ships.