South Korea has retaliated after North Korea opened fire with around 200 artillery shells on a military base situated on the border island of Yeonpyong, killing at least two marines.

Approximately 200 artillery shells were fired at the island of Yeonpyong in one of the most serious clashes since the end of the Korean War, just days after North Korea unveiled a new nuclear-capable uranium enrichment facility.

The South Korean military has been placed on its highest non-wartime alert and has also scrambled fighter jets to protect the island, where around 1,600 residents have been evacuated to shelters.

According to South Korea’s Yonhap news agency, President Lee Myung-bak warned the North that it would “sternly retaliate” against further provocations.

“North Korea’s shelling of Yeonpyeong Island constitutes a clear armed provocation. Furthermore, its reckless shelling of civilian targets is unpardonable,” he said.

The office of the US White House press secretary has strongly condemned the attack and affirmed it is “firmly committed” to the defense of its ally Seoul.

“We are in close and continuing contact with our Korean allies. The US strongly condemns this attack and calls on North Korea to halt its belligerent action and to fully abide by the terms of the Armistice Agreement,” the statement said.

US special representative for North Korea Stephen Bosworth has ruled out the resumption of six-party talks on resolving the nuclear issue, while South Korean presidential spokesman Kim Hee-jung said the country was investigating a possible link between the attack and military exercises carried out near the western sea border.