The Defence Ministry in Seoul said the North Korean vessel crossed the Yellow Sea border just before 7 am (local time Yesterday).

The de-facto maritime boundary between the two Koreas - the Northern Limit Line - is not recognised by Pyongyang, which argues it was unilaterally drawn by US-led United Nations forces after the 1950-53 Korean War.

Both sides complain of frequent incursions by the other and there were limited naval clashes in 1999, 2002 and 2009.

Incidents like today's intrusion are quite common and rarely escalate into anything more serious.

However, South Korea is on a high state of alert following yesterday's rocket launch, which Seoul insists amounted to a disguised ballistic missile test.

South Korean President Park Geun-hye has called for heightened vigilance against any further provocation following the launch and North Korea's fourth nuclear test last month.

Read more: North Korea's rocket launch sparks global outrage

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