South Korean military officials have said that the North fired 100 shells into waters near the disputed inter-Korean sea border. The incident comes after a South Korea accused the North of detaining a fishing boat.
S Korea recently finished its largest ever anti-submarine drill
The shells were fired by North Korea's batteries shortly after the South Korean navy had ended five days of exercises south of their disputed sea border, South Korea's news agency Yonhap reported, citing military officials.
The North had threatened to retaliate against the drills, which was the South's largest ever anti-submarine exercise.
In July, the US and South Korea conducted joint exercises, which North Korea had threatened could start a "war."
South Koreans watch the Nimitz-class USS George Washington leaving for joint military exercises with South Korea
"Our navy was placed on high alert, closely watching the movement of the North Korean troops," a senior official told AFP on Monday. Other officials said that the South's navy patrol ships had warned the North Koreans by radio to stop firing but the warning was ignored.
Earlier this year, the North fired a series of artillery rounds in the same area. Tension has been high in ever since a South Korean warship, the Cheonan, was sunk in March, killing 46 sailors. Seoul accuses Pyongyang of torpedoing its ship. The latter has rejected the allegations.
Seoul accuses Pyongyang of detaining fishing boat
The situation between the two rival neighbors was further exacerbated on Sunday when South Korea accused the North of detaining one of its fishing boats, the Daeseung.
South Korean mourners after the sinking of the Cheonan warship killed 46 sailors
Seoul has called for the boat and crew to be immediately released. A spokesperson for the South Korean Unification Ministry said it was unclear where exactly the boat, with four South Korean and three Chinese crew members, was located when it was seized.
According to the South Korea's coastguard, the fishermen were questioned at sea before being taken towards the North's eastern port of Songjin.
Meanwhile, the Chinese embassy in North Korea has expressed concern over the reported detention of the boat.
Military officers from the North and the US-led United Nations Command are due to meet on Tuesday at the Panmunjom truce village to discuss the sinking of the Cheonan. If it goes ahead as planned, Tuesday's meeting will be the fourth round of such talks.
Editor: Anne Thomas