A North Korean patrol boat crossed into South Korean waters early on Monday and retreated after the South Korean navy fired warning shots, a South Korean military official said.
The incursion came amid heightened tensions on the Korean peninsula, a day after North Korea fired a long-range rocket carrying a satellite into space, a launch that South Korea and other countries consider to be a missile test in disguise.
The patrol boat crossed the Northern Limit Line, which North Korea does not recognise, in the Yellow Sea to the west of the peninsula, at 6:55 a.m. (21:55 GMT) near Socheongdo island, the official said.
Yonhap news agency said the patrol boat crossed despite warning communications from the South Korean navy, and retreated after five warning shots were fired by a naval gun, returning back across the Northern Limit Line at around 7:15 a.m.
North Korea has refused to recognise the so-called Northern Limit Line that was drawn up at the end of the 1950-53 Korean War and has frequently challenged it with intrusions of ships, and more recently by firing artillery near or across the line.