North and South Korea have exchanged artillery fire near their disputed sea border.
The incident highlights recent instability along the heavily armed frontier.
North Korea warned the South that more rounds were on the way as a part of military training.
It then fired off another barrage a few hours after delivering the message in a state media report.
Analysts doubt the latest clash will escalate, seeing it more as an attempt by Pyongyang to stress tensions on the Korean peninsula and press home its demand for a peace deal that would open the way to international aid for its ruined economy.
South Korea's Joint Chiefs of Staff said the North fired artillery from land towards the South, which landed on its side of the disputed sea border off the west coast.
South Korea returned fire from its coastal artillery.
'We want to express grave concern over the incident that resulted from the North's illegal act that unnecessarily creates tension through live-fire artillery fire,' the South's Defence Ministry said in a message to the North.
South Korea's Yonhap news agency said the North probably fired about 30 rounds of artillery and Seoul responded with about three times the number.
The firing came when President Lee Myung-bak was travelling to Davos in Switzerland for the World Economic Forum after a state visit to India.
His office was quoted by Yonhap as saying both sides fired into the air and there were no casualties.
Earlier this week Pyongyang accused the South of declaring war by saying it would launch a pre-emptive strike if it had clear signs the North was preparing a nuclear attack.
The latest clash also comes amid signals from Pyongyang it was ready to return after a year-long boycott to six-country talks - between the two Koreas, China, Japan, Russia and the US - on ending its atomic arms programme.
In return for resuming disarmament negotiations, North Korea has demanded talks on a peace treaty with the US to finally end the 1950-53 Korean War and the lifting of UN sanctions over its two nuclear tests.
