South Korea scrambled naval vessels on Wednesday after a number of North Korean fishing boats violated their disputed Yellow Sea border, a coastguard spokesman said.
Six to seven North Korean boats were spotted operating illegally in South Korea-controlled waters near Yeonpyeong island, the spokesman said.
South Korean naval ships were immediately deployed to escort the vessels back across the maritime border, he said.
The South’s defence ministry declined to comment on the incident.
Yonhap news agency said the South’s navy spotted the vessels at around 11.10am (10.10am Hong Kong time) about three kilometres south of the border line, and ordered them to return to their territory.
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.