North and South Korea have exchanged fire across the disputed western maritime border, South Korea has said.
North Korea announced early on Monday that it would hold live-fire drills in seven parts of the border area, BBC reported.
South Korea says it returned fire after North Korean shells landed in its territorial waters.
The area has been a flashpoint between the two Koreas. In late 2010, four South Koreans were killed on a border island by North Korean artillery fire.
Earlier that year, a South Korean warship sank near the disputed border with the loss of 46 lives.
Seoul says Pyongyang torpedoed the vessel but North Korea denies any role in the incident.
The UN drew the western border after the Korean War, but North Korea has never recognised it.
The live-fire exercises were announced by North Korea in a faxed message from its military to the South's navy.
The exercises would take place in seven border areas, the message said.
South Korean had warned of immediate retaliation if any ordinance crossed the border.
In November 2010, North Korea fired shells at the border island of Yeonpyeong, killing two marines and two civilians.
It said it was responded to South Korean military exercises in the area.
The latest incident comes days after North Korea test-fired two medium-range Nodong missiles over the sea, its first such launch since 2009.
Late last week, the UN Security Council condemned the launch and said it was considering an "appropriate response".
That launch followed a series of short-range missile tests, seen as a response to the current US-South Korea annual military exercises.
Over the weekend, North Korea also threatened to conduct a "new form" of nuclear test.