North Korea says the firing is part of a regular military drill
North Korea has declared four naval firing zones near its disputed sea border with South Korea, South Korea's Joint Chiefs of Staff said.
The South Korean news agency Yonhap says the North has deployed multiple rocket launchers close to the frontier.
The "no sail" zones suggest the North might conduct firing exercises over the next three days.
It fired artillery in the disputed seas in late January, amid international efforts to re-start nuclear talks.
The communist state declared its exclusion zones effective from Saturday.
They are in the Yellow Sea - including two near the border - and two in the Sea of Japan (East Sea) off its northeast coast, South Korea's Joint Chiefs of Staff said.
The Yellow Sea border was the scene of deadly naval battles in 1999 and 2002 and of a fire fight last November which left a North Korean patrol boat in flames.
Yonhap news agency said defence ministry officials gave the information Friday to a closed meeting of a parliamentary defence committee.
"North Korea has deployed multiple rocket launchers in the western coastal bases since the naval clash in November," a military official was quoted as telling Yonhap.
After declaring two "no sail" zones, the North in late January fired 370 shells into the sea near the border over three days.
The North said it was staging a routine exercise but South Korea and the United States described the firing as provocative.
Since then the North has again declared "no sail" zones but not gone ahead with any exercises.
North Korea is under pressure to return to disarmament talks, and some analysts say the firing exercises may be a way to use military threats to help strengthen its hand to win concessions from regional powers.
The western sea border is a constant source of military tension between the two Koreas. There have been three deadly exchanges between them in the past decade.
South Korea recognises the Northern Limit line, drawn unilaterally by the US-led United Nations Command to demarcate the sea border at the end of the 1950-53 Korean War. The line has never been accepted by North Korea.
The US, China, Russia, Japan and South Korea speak regularly of their hopes that North Korea will rejoin international talks about ending its nuclear programme.
News reports suggest a North Korean nuclear envoy Kim Kye Gwan will be visiting the United States shortly, after being in China for talks.
North Korea has also re-stated its opposition to new talks until the US changes what the North calls its "hostile policy".