South Korea scrambled naval vessels after seven North Korean fishing boats violated their disputed Yellow Sea border, authorities said.
Seven North Korean boats were spotted operating illegally in South Korea-controlled waters near Yeonpyeong island, the defence ministry said on Wednesday.
"Our navy was put into position to take quick action," a ministry spokesman told AFP, adding that a patrol ship broadcast warning messages ordering the North Korean vessels to go back.
The North Korean boats crossed the border twice at 7:30 am (2230 GMT) and two hours later, but had all moved back into the North's waters by midday, he said.
It was the second such incident involving North Korean fishing boats this year.
Yeonpyeong was bombarded by North Korea's artillery in November 2010. The attack killed four South Koreans and briefly sparked fears of a full-scale conflict.
The Yellow Sea border drawn by United Nations forces after the 1950-53 Korean War was the scene of deadly naval clashes in 1999, 2002 and 2009. Pyongyang refuses to accept the line.
The South has strengthened manpower and weaponry on its frontline islands to forestall any fresh attack.
The two Koreas have remained technically at war since the Korean War was concluded with a truce rather than a peace treaty.