North Korea has fired artillery shots near the disputed sea border with South Korea and the South has returned fire, according to Yonhap news agency.
It said the North fired several shells into the sea near the South Korean-controlled Baengnyeong Island off the countries' western coast.
On Tuesday, North Korea declared a no-sail zone in waters off its coast, media reports say.
South Korea officials said the exchange caused no casualties or damage.
The North fired into waters near the border just after 0900 local time (0000 GMT), a spokesman for the Joint Chiefs of Staff told AFP.
"Our military immediately fired back in response," a Seoul presidential official told the news agency on condition of anonymity.
Constant tension
The official said the North's artillery rounds landed north of the sea border, while Seoul's forces fired at the rounds while they were in the air, AFP reports.
The western sea border is a constant source of military tension between the two Koreas.
There have been three deadly exchanges between the two Koreas along the sea border 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 at the end of the 1950-53 Korean War, which has never been accepted by North Korea.
Talks off
Recent talks between the two Koreas 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 the North's leader Kim Jong-il dies.
North Korea did recently accept a small amount of aid from South Korea however.
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.
The (disputed) sea line between North and South Korea.
North Korea and South Korea have exchanged fire near the disputed sea-border between the countries. There are no reports of casualties.

North Korea fired approximately thirty artillery shells into the water on their side of the border, near South Korea's Baengnyeong Island at 0900 local time (0000 UTC). The South retaliated with around 100 shots. 

According to the North, it took place as part of an annual drill. The Korean Central News Agency said that the exercise "will go on in the same waters in the future".

South Korea maintains that it responded by firing at the North's shells. Later reports from the South said that North Korea fired more shells in the general direction of the disputed border some hours after the initial exchange.

This latest incident comes after North Korea declared a 'no-sail' zone off its coastal waters yesterday.  It is not the first skirmish around the Military Demarcation Line – the most recent was in November 2009, when a North Korean vessel took "considerable damage" after allegedly crossing the border.