A South Korean naval vessel has fired warning shots after two North Korean patrol boats crossed the disputed maritime border, just before United States President Barack Obama arrived in Seoul for a two-day visit.
The North Korean boats, which normally serve to keep fishing boats on the right side of the boundary, crossed "one nautical mile south" into South Korean waters on Friday, a defence ministry official said.
The pre-dawn incursion prompted a South Korean naval ship to fire several warning rounds, after which the two vessels retreated to the North side of the border.
The official said the patrol boats might have been chasing some Chinese crabbing boats fishing illegally in the area. "Or the North might have wanted to check the South's military vigilance," he added.
The North does not officially recognise the Yellow Sea boundary, which was unilaterally drawn by the US-led United Nations forces after the 1950-53 Korean War, AFP reports.
The border has been 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.
It is not rare for North Korean patrol boats and fishing boats to cross the unmarked sea border into the South, but the timing was sensitive on Friday with Obama's arrival.
Tensions are high on the Korean peninsula, with multiple indications that North Korea might be planning to carry out a fourth nuclear test. Others were in 2006, 2009 and 2013.