North Korea says the firing is part of a regular military drill
North Korea has fired artillery shells near its disputed maritime border with South Korea for a third successive day.
Pyongyang says the firing is part of an annual military drill but Seoul says it is "provocative" and on Wednesday fired retaliatory warning shots.
South Korean President Lee Myung-bak told the BBC the North was using the firing as a negotiating tactic.
But he said he was still willing to meet North Korean leader Kim Jong-il to discuss the country's nuclear plans.
North Korean ships fired about 20 artillery rounds into the sea around the disputed western border on Friday, South Korean reports said.
The Joint Chiefs of Staff in Seoul said the shells appeared to have landed in the North's waters, Yonhap news agency reports.
Spokesman Park Sung-woo said Seoul would be "closely observing the situation throughout the day".
Yonhap said the South's defence ministry was considering positioning an artillery-tracking radar system in the area.
The third maritime firing incident in as many days comes after the North designated two no-sail zones in the area, including some South Korean waters, until 29 March.
On Wednesday, South Korean coastal bases fired warning shots after the North fired shells into the sea.
North Korea fired for a second time later that day and again on Thursday, but the South has not retaliated.
Mr Lee said there could be many reasons why North Korea had taken such action at this time.
Lee Myung-bak said any talks would have to be "candid"
But he said the firing could be a "negotiating tactic", in response to international pressure on Pyongyang to return to six-party talks on its nuclear programme.
Me Lee said he could meet North Korea's Kim Jong-il later this year if conditions were right, but that he did not want to "place any definitive time" on a summit.
"I've always thought to myself that if I were to meet personally with Chairman Kim that our dialogue and discussions must be constructive," he said.
Mr Lee said such talks must "yield actual results" and that there should be no preconditions to meeting.
"We must be able to talk openly and candidly about the North Korean nuclear issue."
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.
In the most recent incident, last November, their navies fought a brief gun battle that left one North Korean sailor dead and three others wounded.
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.
Relations between the two Koreas have fluctuated in recent months. Talks about their jointly run Kaesong industrial estate closed without agreement on 21 January.
The attempt at dialogue took place amid fresh tensions apparently provoked by a South Korean think tank's analysis of a likely military coup or mass uprising in the North when Kim Jong-il dies.
However, North Korea did recently accept a small amount of aid from South 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.