A North Korean patrol boat reportedly crossed the country's maritime border with South Korea in the Yellow Sea on Monday.
South Korea's military fired five warning shots at the boat, following which it retreated, according to Yonhap News Agency.
The incident, which took place at 6:55 a.m. local time, comes a day after North Korea launched a long-range rocket to put a satellite in space.
The Kim Jong-un regime was strongly criticised following the launch, which critics believe is a cover for testing technology for a long-range missile that can carry nuclear warheads. The United states Security Council condemned the rocket launch.
Last month, North Korea claimed it had successfully tested a hydrogen bomb.
The maritime border between North and South Korea, called the Northern Limit Line (NLL), was declared by the United States-led United Nations Command after the two sides entered a ceasefire following the 1950-53 Korean War. The disputed maritime border has witnessed clashes in 1999, 2002 and 2009.
South Korea has been on a state of alert since the Kim Jong-un regime's declaration of testing a hydrogen bomb and carrying out a rocket launch.