The shots were fired just after the top U.S. commander in South Korea, Gen. Walter Sharp, toured the island of Yeonpyeong in a show of solidarity with Seoul.
Tensions have risen between the Koreas since the North's strike Tuesday destroyed large parts of Yeonpyeong, killing two civilians and two marines in a major escalation of their skirmishes along the sea border.
The skirmish forced South Korea's defense minister to resign Thursday, and President Lee Myung-bak on Friday named a former chairman of the Joint Chiefs of Staff to the post.
The heightened animosity between the Koreas is taking place as the North undergoes a delicate transition of power from leader Kim Jong Il to his young, inexperienced son Kim Jong Un, who is in his late 20s and is expected to eventually succeed his ailing father.
The Associated Press contributed to this story.