orth and South Korea exchange fire near sea border

North Korea has fired artillery shots near the disputed sea border with South Korea and the South has returned fire. The North fired shells into the sea near Baengnyeong Island off the the South's western coast, South Korean news agency Yonhap said. North Korea said the firing was part of an annual military drill, adding that it would continue. 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 (2400 GMT), a spokesman for South Korea's 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. A statement carried by the official Korean Central News Agency says its drills "will go on in the same waters in the future, too". Just hours after the initial exchange, reports from South Korea said that the North had fired more shells in the direction of the disputed border. It is not clear where they landed. Constant tension The South Korean official said the North's initial 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. The BBC's John Sudworth in Seoul says this latest exchange is being interpreted as another attempt by North Korea to increase tension and stress the instability on the peninsula, thus helping it to gain diplomatic concessions. Talks off 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. 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.