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.

South Korea's Defence Ministry had no immediate comment on the fresh warning. 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 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.

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