S. Korea fires warning shots after N. Korean incursion
A South Korean Navy vessel (R) sails by a fishing boat (L) off the South Korea-controlled island of Yeonpyeong near the disputed waters of the Yellow Sea on April 12, 2013 - by Jung Yeon-Je
The North Korean boats, which normally serve to keep fishing boats on the right side of the boundary, crossed into South Korean waters at 0700 GMT, the South's joint chiefs of staff said.
The incursion prompted a South Korean naval ship to fire 10 warning rounds, after which the North Korean vessels retreated to the North side of the border, it said.
Yonhap news agency said the North Korean boats might have been chasing some Chinese boats fishing illegally in the area.
It is not uncommon for North Korean patrol boats and fishing boats to cross the unmarked sea border into the South.
Two North Korean patrol boats violated the sea border last month, just before US President Barack Obama arrived in Seoul for a two-day visit.
The North does not officially recognise the Yellow Sea boundary, the scene of brief but bloody naval clashes in 1999, 2002 and 2009.
The Korean conflict ended in an armistice instead of a peace treaty and technically the two Koreas are still at war.
In March, North Korea fired hundreds of shells in a live exercise near the sea boundary. About 100 shells dropped into South Korean territorial waters, and South Korea responded with volleys of shells into North Korean waters.
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