North Korea has designated another two "firing zones" near its disputed Yellow Sea border with South Korea, raising the prospect of more artillery fire, Seoul officials said Wednesday.

A Joint Chiefs of Staff spokesman told AFP the two new "maritime firing zones" would be effective for four days from Friday.

The spokesman said the South's military is closely watching for possible artillery fire off Baengnyeong and Daecheong islands near the Northern Limit Line sea border.

After declaring two "no sail" zones, the communist state last week fired 370 shells into the sea near the border over three days, heightening tensions on the Korean peninsula.

The North said it was staging a routine exercise but South Korea and the United States described the firing as provocative.

The Yellow Sea border was the scene of deadly naval battles in 1999 and 2002 and of a firefight last November which left a North Korean patrol boat in flames.