The South Korean navy has fired warning shots at a North Korean patrol boat which had crossed the disputed maritime border between the two Koreas this morning.
The incident came amid increasing jitters in South Korea in the wake of a US revelation that North Korea had admitted pursuing a nuclear arms development programme, violating the 1994 accord, known as the Agreed Framework.
The North Korean boat infringed the Northern Limit Line off the western coast just before 6 a.m. (Irish time) and returned to the North 14 minutes later after the South's warning shots, a Defence Ministry official said.
He did not elaborate further. Washington and its allies decided last week to stop vital fuel oil aid to penalise Pyongyang for breaking a series of nuclear non-proliferation pledges. The cuts will hit North Korea just ahead of winter, which brings sub-freezing temperatures.
North Korea has not yet responded to the decision to cut the fuel shipments - a move Pyongyang envoys have said would be viewed as a hostile act.
Under a 1994 agreement, the North promised to freeze its nuclear weapons programme in return for fuel oil, paid for by Washington, and two light water reactors that cannot easily be converted to produce atomic weapons material.
North Korea has never recognised the Northern Limit Line, an extension in the Yellow Sea of the land border between the rival states at the end of the 1950-53 Korean War.
The two Koreas remain in a technical state of war because the 1953 armistice which stopped the shooting has never been replaced by a peace treaty.